Fiscal computation¶

The engine computes the full gross-to-net chain: INPS, IRPEF, trattamento integrativo, and optionally regional and municipal surcharges.

See Domain: IRPEF and surcharges for the legal background.

The tax year of a run follows its payment date, with the 12 January extension of TUIR art. 51 c. 1: see Tax year and payment date.

Some pay items take a flat substitute tax instead of IRPEF when the worker meets the statutory requirements: see Substitute-tax regimes.

The regional and municipal surcharges are described in Surtax; the decisions of the ulteriore detrazione and the trattamento integrativo in Tax credits.

IRPEF flow¶

1. Taxable income = gross − INPS employee contributions
2. IRPEF gross = taxable income × progressive brackets (Art. 11 TUIR)
3. Work income deduction (Art. 13 TUIR) reduces IRPEF gross. Its income ratios are truncated to four decimals (art. 13 c. 6 TUIR). Up to €15,000 it is at least €690, or €1,380 for a fixed-term employment (c. 1 lett. a), proportioned to the days in the withholding; apprenticeship is open-ended (D.Lgs. 81/2015 art. 41 c. 1).
4. Ulteriore detrazione lavoro (Art. 1 c. 6 L. 207/2024): additional credit of up to €1,000/year for taxable income between €20,000 and €40,000. Flat €1,000 from €20,001 to €32,000; linear taper to zero from €32,001 to €40,000. The taper ratio is not truncated: c. 6 has no four-decimal rule and the one of art. 13 c. 6 TUIR lists the ratios of art. 13 only. The income is the reddito complessivo: this employment plus what current_year states beyond it, with the exempt share of the impatriati and researcher regimes (c. 9). While it is due, a run without current_year of its tax year has the issue ulteriore_income_unknown (fact="current_year").
5. IRPEF net = IRPEF gross − work income deduction − ulteriore detrazione − family deductions + clawback sterilisation (floored at 0). Art. 15 TUIR deductions are not applied in payroll (capability art15_deductions, outside the input): the worker claims them in the tax return.
6. Trattamento integrativo (Art. 1 D.L. 3/2020): up to €1,200/year. Two income bands apply:
7. RC ≤ €15,000 (c. 1, first period): granted when IRPEF gross exceeds the Art. 13 deduction minus a €75 corrective for the days of work (words inserted by L. 207/2024).
8. €15,001–€28,000 (c. 1, second and third periods): granted only when the sum of the Art. 12 and Art. 13 c. 1 deductions exceeds IRPEF gross; the amount equals the excess, capped at €1,200. The ulteriore detrazione is not in the list. The law also counts Art. 15 deductions for loans taken out up to 31 December 2021 and the instalments of expenses incurred up to that date; payroll does not know them, so the credit of the run can only be lower than the one of the tax return.
9. RC > €28,000: zero.

For a part-year employment the work deduction, the ulteriore detrazione and the trattamento integrativo (with its €75 corrective) are "rapportata al periodo di lavoro nell'anno": the full-year amount, in cents, times days / 365 (730/2026 istruzioni, quadro C: "365 per l'intero anno"). The days are those "per i quali spetta la retribuzione" (AdE circ. 15/E/2007 par. 1.5.1): the days of an AbsenceEvent with no_pay_due=True (aspettativa senza assegni) leave them, a strike (no_pay_due=False) does not, and an absence that leaves it None has the issue deduction_days_unknown. The spells of the tax cash state keep the unpaid days (EmploymentSpell.unpaid_days). The day ratio is not truncated to four decimals, since it is not one of the ratios art. 13 c. 6 TUIR lists: 200 days of the €1,955 deduction give €1,071.23, not €1,071.14.

The minimum of art. 13 c. 1 lett. a) is proportioned to the days in the withholding. The Certificazione Unica 2026 instructions of the Agenzia delle Entrate (updated 24 February 2026, punto 367, p. 33, PDF, sha256 a6ccf7cf53edcbd0d084c2266868649f8d17c348644401b540efd5e3fc95e841) read: "Nel caso di rapporti di lavoro a tempo determinato o a tempo indeterminato di durata inferiore all'anno (inizio o cessazione del rapporto di lavoro nel corso dell'anno), limitatamente ai redditi di cui ai punti 1 e 2, il sostituto deve ragguagliare anche la detrazione minima al periodo di lavoro". The worker is told, with code AN (Tabella F, p. 93), that the tax return grants the deduction for the whole year: there the minimum "non deve essere rapportata ai giorni di lavoro dipendente" (Allegato C to the 730/2026 instructions, par. 19.9.1). The deduction of the withholding is the larger of the two amounts for the days. With the 2026 amounts the minimum for the days never exceeds €1,955 for the days, so up to €15,000 the withholding deducts €1,955 × days / 365: 92 days give €492.77, against €173.92 (690 × 92 / 365) or €347.84 (1,380 × 92 / 365). This applies to every withholding of the year, the conguaglio of a termination included, and the trattamento integrativo test up to €15,000 compares the gross tax with that deduction less the €75 corrective for the days. Those instructions certify 2025 income; the engine reads the rule as unchanged for 2026.

When the whole minimum exceeds the deduction of the withholding, the run records an irpef decision with reason minimum_proportioned_to_days, no amount, and the inputs minimum and tax_return_balance, the part the tax return grants: Metalmeccanico C3 from 10 July to 20 September 2026 (73 days) deducts €391.00 and leaves €299.00 (€690 − €391.00) or, fixed-term, €989.00 to the tax return. The minimum is €1,380 when any employment the withholding counts in the year is fixed-term, as the 730 takes it when "in almeno un rigo" of C1 to C3 holds code 2 (Allegato C, par. 19.9.1).

The withholding agent may grant the whole minimum when the worker asks for it (punto 367: "sempreché non sia già stata attribuita, su richiesta del percipiente, dallo stesso sostituto"; punto 473 code B, p. 40). The engine has no input for that request and always proportions the minimum.

The days are those of every employment whose income the withholding projects. A rehire by the same employer in the same tax year, whose first run opens with the state the earlier employment closed, projects the income of both, so it counts the days of both: one Certificazione Unica holds "più rapporti di lavoro ... per il medesimo periodo d'imposta" (punto 11 code 1 for an employment "interrotto e successivamente ripreso"), and punto 721 counts the days of "tutti i rapporti di lavoro conguagliati", "i giorni compresi in periodi contemporanei" once (istruzioni CU 2026, updated 24 February 2026). The tax cash state records each employment as an EmploymentSpell of the tax year (TaxCashState.employment_spells): its first day, its last day (31 December while the end is not stated) and whether it is fixed-term; a later run of the same employment, keyed by its first day, replaces it. Metalmeccanico C3 from 1 January to 31 March 2026, rehired on 1 June, counts 90 + 214 = 304 days from June.

A termination run closes the employment: no run but an adjustment closes after it in its competence year, and a rehire after it is refused with an InvalidInputError that names the termination run. Open the rehire with PeriodState.zero(): its withholding then counts its own income and days only. Art. 23 c. 4 DPR 600/1973 (text in force in 2026) lets the worker ask the year-end conguaglio to count the income "percepiti nel corso di precedenti rapporti intrattenuti"; the engine does not take that income as an input.

Withholding of a run¶

Art. 23 c. 2 DPR 600/1973 (in force for 2026) withholds on each payment; every run also projects the annual taxable income (YTD, this run, and the recurring pay of the slots still to come), the reddito complessivo the deductions are measured on (AdE circ. 15/E/2007 par. 2.1: without other indication, the employment income the employer "nel corso dell'anno corrisponde"):

• a regular month (lett. a) withholds on the taxable of the period "con le aliquote dell'imposta sul reddito delle persone fisiche, ragguagliando al periodo di paga i corrispondenti scaglioni annui di reddito, ed effettuando le detrazioni previste negli articoli 12 e 13 ... rapportate al periodo stesso": the brackets divided by twelve, less the art. 13 deduction and the ulteriore detrazione of the year times the days of the month over the days of employment in the year (circ. 15/E/2007 par. 1.5.1 and 2.3), and less a twelfth of the art. 12 deduction of each dependant whose conditions hold in that month (art. 12 c. 3 TUIR: "dal mese in cui si sono verificate"). A deduction above the tax of the month is lost to the month; the conguaglio settles the year;
• a tredicesima or quattordicesima (lett. b, "sulle mensilità aggiuntive e sui compensi della stessa natura ... ragguagliando a mese i corrispondenti scaglioni annui di reddito") is taxed on the same monthly brackets with no deduction. So is a bonus or a productivity bonus taxed ordinarily, and the PdR above its cap, on the payslip that pays it, apart from the pay of the month: circ. 15/E/2007 par. 2.4 counts among those compensi "i cosiddetti premi trimestrali, semestrali e annuali". Its taxable is the premium less the employee INPS at the rate of the run. Overtime, shift and holiday pay, current-year arrears, renewal increments and fringe benefits are pay of the period (lett. a). A €20,000 bonus paid in November to a Metalmeccanico C3 withholds €7,509.01 on the November payslip: €7,133.86 on the bonus and €375.15 on the rest of the month;
• an adjustment run before the conguaglio withholds on its own taxable, with no deduction (the month already took them);
• the payment that leaves no slot unpaid (the conguaglio) settles the whole balance on the final income, which can be a refund (art. 23 c. 3). The slots are the payments actually made in the tax year, so with December paid on 13 January the conguaglio of the year falls on the tredicesima, and a payment made after the conguaglio settles the year again (see Payroll state).

Metalmeccanico C3 in January 2026: taxable 1,953.45, 23% = 449.29; the year is projected at 25,394.74, art. 13 2,213.48 x 31/365 = 187.99, ulteriore detrazione 1,000 x 31/365 = 84.93; January withholds 176.37. Commercio level 4 hired on 1 January: the June quattordicesima (6/12, taxable 809.92) withholds 23% of it, 186.28, while the June regular month withholds 87.25.

The trattamento integrativo is "ripartendolo fra le retribuzioni erogate" (D.L. 3/2020 art. 1 c. 3): it pays the annual amount still due divided by the slots not yet paid, the tredicesima included. The somma esente is recognized "all'atto dell'erogazione delle retribuzioni" (L. 207/2024 art. 1 c. 7) by "applicando tale percentuale al reddito effettivamente corrisposto mensilmente" (AdE circ. 4/E/2025 par. 1.2): each run pays the percentage of the projected annual income times the taxable it pays (somma_esente_period in tax_computation.components), capped at what is still due, and the conguaglio settles the year. Commercio level 4 hired on 1 July 2026: July pays 4.8% of 1,619.82, 77.75.

The somma esente is due when the reddito complessivo is not above 20,000 EUR (c. 4). It counts this employment, the income current_year states (net of the main dwelling) and CurrentYearTaxFacts.exempt_regime_income, the exempt share of the researcher (art. 44 c. 1 D.L. 78/2010) and impatriati (art. 16 D.Lgs. 147/2015, art. 5 D.Lgs. 209/2023) regimes, which c. 9 adds; the family deductions do not count that share.

The projection of a future tredicesima or quattordicesima uses the rateo the run will pay on the employment period: a worker hired on 1 July is projected 6/12 of the tredicesima, not a full month. Absences still to come are not known and settle at the conguaglio.

Pay that does not cover the tax¶

A run can owe more IRPEF and surtax than the pay it leaves after the contributions and the other deductions, for example when unpaid absences take most of the month. The engine then:

• withholds IRPEF first, up to the pay left, and the surtax from what remains, so the net pay is never negative because of the taxes; a conguaglio refund is never capped;
• carries what it could not withhold in state.cash.shortfall (irpef, surtax) and withholds it in full on the next run, on top of the IRPEF of that run's own pay period;
• records a withholding_shortfall decision (withholding_capped when it carries an amount out, shortfall_withheld when it withholds a carried amount), with the pay available and the amounts due in its inputs;
• on the last withholding slot, reports what is still not withheld as a provisional withholding_shortfall_unrecovered issue.

The other deductions are not capped. The employee INPS share is withheld "sulla retribuzione corrisposta al lavoratore stesso alla scadenza del periodo di paga cui il contributo si riferisce" (L. 218/1952 art. 19), and carrying it, a substitute tax or a recovery installment to a later payslip is not modelled. A run whose pay does not cover them, for example a 20,000 EUR fringe benefit on a part-time Commercio level 7 in March 2026, raises OutOfScopeError with reason negative_net (feature net_pay), never a result with a negative net.

The cumulative conguaglio settles the tax on the whole year (art. 23 c. 3 DPR 600/1973, in force for 2026: Normattiva gives it "in vigore dal 21-5-2022 al 31-12-2026"; the same text is art. 33 c. 4 D.Lgs. 33/2025, in force from 1 January 2027 by art. 243 as amended by D.L. 200/2025 art. 4). For what is left at year end the same comma says: "L'importo che al termine del periodo d'imposta non è stato trattenuto per cessazione del rapporto di lavoro o per incapienza delle retribuzioni deve essere comunicato all'interessato che deve provvedere al versamento entro il 15 gennaio dell'anno successivo". The worker can instead ask in writing to have it withheld on the next pay periods (see Written deferral of the year-end shortfall). The statute does not state how a shortfall found before the conguaglio is spread: taking it on the next run is the engine's choice. Art. 23 c. 1, second sentence (art. 33 c. 1 D.Lgs. 33/2025 from 2027), which makes the worker pay the withholding that finds no cash, is written for values in kind and is not used here.

Metalmeccanico C3 in January 2026, 100 absence hours at 12.50 EUR and a 3,000 EUR fringe benefit in kind above its threshold: the cash left after INPS is 537.37 EUR, the IRPEF of the month is 673.40 EUR; January withholds 537.37 EUR and nets 0.00, February withholds the 136.03 EUR carried on top of the IRPEF of its own month.

Credit recoveries (trattamento integrativo, somma esente, installments of an earlier year) are capped in the same way and taken before the IRPEF: the part the pay does not cover is given back by a credit_recovery_shortfall_{run_id} line and carried as shortfall.credit_recovery, apart from the IRPEF. On the last run of the employment it is reported by the same provisional issue; AdE circ. 29/E/2020 par. 6 and 4/E/2025 par. 1.2 apply art. 23 c. 3 DPR 600/1973 to a recovery the conguaglio di fine rapporto cannot make "per incapienza della retribuzione".

A run whose other deductions (INPS, substitute tax) exceed the pay left by unpaid absences is still rejected (OutOfScopeError, reason withholding_shortfall).

Written deferral of the year-end shortfall¶

Art. 23 c. 3 DPR 600/1973, second and third sentences (identical in art. 33 c. 4 D.Lgs. 33/2025 from 2027):

In caso di incapienza delle retribuzioni a subire il prelievo delle imposte dovute in sede di conguaglio di fine anno entro il 28 febbraio dell'anno successivo, il sostituito può dichiarare per iscritto al sostituto di volergli versare l'importo corrispondente alle ritenute ancora dovute, ovvero, di autorizzarlo a effettuare il prelievo sulle retribuzioni dei periodi di paga successivi al secondo dello stesso periodo di imposta. Sugli importi di cui è differito il pagamento si applica l'interesse in ragione dello 0,50 per cento mensile, che è trattenuto e versato nei termini e con le modalità previste per le somme cui si riferisce.

The request is PriorYearTaxFacts.shortfall_deferral, a ShortfallDeferralRequest(signed_on=...) signed between 1 January of the tax year and the end of February of the next one (the deadline of the conguaglio); another date raises InvalidInputError. Without it nothing changes: the shortfall is communicated to the worker with the provisional issue above. With it, on the conguaglio of year N:

• the IRPEF the pay cannot cover becomes a DeferredShortfall in state.cash.obligations.deferred_shortfall, with the date of the request and the pay period of the conguaglio; state.cash.shortfall.irpef is zero and a shortfall_deferral decision shortfall_deferred records the amount. close_tax_year carries it into N+1;
• only the IRPEF is deferred, the tax the norm names; surtax and credit recoveries the pay cannot cover keep the provisional issue;
• when the conguaglio is the last run of the employment no payslip follows: the decision is deferral_not_possible and the issue stays.

On the payslips of N+1 the engine reads "successivi al secondo" as from the March pay period (January and February are the two pay periods on which the conguaglio of N can still be made), and "dello stesso periodo di imposta" as within N+1. The norm fixes no installments: every run from March, adjustment runs excepted, withholds from its net pay, after every other line, the largest principal whose interest still fits, until the amount is exhausted. The interest is simple: 0.50% for each whole month from the pay period of the conguaglio to the pay period of the run, on the principal the run withholds. The norm does not say when the count starts; this is the engine's reading, and each deferred_shortfall_withheld decision records the principal, the interest, the months and the rate.

Both amounts are posted to ORDINARY_TAX on the lines deferred_irpef_{N}_{run_id} and deferred_irpef_{N}_interest_{run_id}, coded 1066 "Ritenute [...] operate dopo il relativo conguaglio di fine anno", instituted by ris. AdE 6/E/2021 for the withholding of art. 23 c. 3 second sentence, with N as the reference year. The interest takes the code of the IRPEF it refers to because the norm remits it "con le modalità previste per le somme cui si riferisce"; no act gives it a code of its own. Neither amount enters the IRPEF withheld of N+1 (state.cash.tax.irpef).

What the conguaglio of N+1 or the last run of the employment still leaves is dropped with a provisional deferred_shortfall_unrecovered issue and decision: it is communicated to the worker as any other shortfall.

A later run of N that settles the balance again (an adjustment run) counts the deferred IRPEF as withheld, so it withholds only the tax of its own pay. Such a run cannot refund IRPEF while the deferral is open: lowering the deferral by the refund is not modelled and raises OutOfScopeError (reason shortfall_deferral_refund). A termination run of N after the conguaglio drops the deferral with the same provisional issue, since no payslip of N+1 follows.

A 300.00 EUR deferral of the December 2025 conguaglio, withheld on the March 2026 payslip: 3 months, interest 300.00 × 0.50% × 3 = 4.50 EUR, net pay 304.50 EUR lower, F24 code 1066 for 304.50 EUR. When March absences leave no pay, April withholds it with 4 months: 6.00 EUR.

Foreign tax credit at the conguaglio¶

Art. 23 c. 3 DPR 600/1973, last sentences (art. 33 c. 4 D.Lgs. 33/2025 from 2027): "Se alla formazione del reddito di lavoro dipendente concorrono somme o valori prodotti all'estero le imposte ivi pagate a titolo definitivo sono ammesse in detrazione fino a concorrenza dell'imposta relativa ai predetti redditi prodotti all'estero. [...] Se concorrono redditi prodotti in più Stati esteri la detrazione si applica separatamente per ciascuno Stato." The withholding agent applies the credit of art. 165 TUIR itself, at the conguaglio.

The input is PriorYearTaxFacts.foreign_taxes: one ForeignTaxPaid(country, income, tax) per State, with the foreign-source employment income as it entered the Italian taxable income of the year (the conventional pay when art. 51 c. 8-bis TUIR applies) and the foreign tax paid "a titolo definitivo", within the treaty rate. When the income entered the taxable income only in part (the conventional pay), pass the tax already reduced as art. 165 c. 10 TUIR requires (circ. AdE 9/E/2015 par. 5). Two entries of one State are rejected.

On the conguaglio only (the last withholding slot or the last run of the employment; the runs before it withhold on the art. 12 and 13 deductions alone, art. 23 c. 2), per State:

quota  = imposta lorda × min(1, income / taxable income)   (to the cent)
credit = min(foreign tax, quota)

and the credits of all States together at most the imposta netta (art. 165 c. 1 TUIR; circ. AdE 9/E/2015 par. 3.1, LIMITE 1 and LIMITE 2). The quota is on the imposta lorda as in the Redditi PF 2026 instructions (fascicolo 3, quadro CE, sezione I-A): the credit "spetta fino a concorrenza della quota d'imposta lorda italiana corrispondente al rapporto tra il reddito prodotto all'estero ed il reddito complessivo [...] e sempre comunque nel limite dell'imposta netta italiana", with the ratio brought back to 1. The withholding agent knows only the income it pays, so the reddito complessivo is the annual taxable income of the conguaglio. The credit is a foreign_tax_credit component of the tax computation, deducted from the net IRPEF of the year, and a foreign_tax_credit decision (credit_applied, or limited_to_net_tax when the imposta netta caps it) with the income, tax, quota and credit of each State in its inputs.

Metalmeccanico C3 2026, 10,000 EUR earned in France and 500 EUR of French tax: taxable income 25,779.81 EUR, imposta lorda 23% of it, quota 5,929.36 × 10,000 / 25,779.81 = 2,300.00 EUR, credit 500 EUR; the IRPEF of the year is 2,751.23 − 500 = 2,251.23 EUR. With the whole income taxed in France at 20,000 EUR the quota is the whole imposta lorda, the credit the imposta netta: no IRPEF and no surtax is due, and the conguaglio refunds what was withheld.

Not modelled, left to the tax return: foreign income taxed in Italy in an earlier year (art. 165 c. 7 TUIR, which the art. 23 rule also allows at the conguaglio), the carry-over of the excess foreign tax (art. 165 c. 6) and a credit above the imposta netta (art. 11 c. 4 TUIR).

Employers that are not withholding agents¶

Everything above is done by the employer as sostituto d'imposta. A household employer is not one (art. 23 c. 1 DPR 600/1973; art. 33 c. 1 D.Lgs. 33/2025 from 2027), so for the domestic CCNLs the engine withholds no IRPEF or surcharge, runs no conguaglio, pays no trattamento integrativo, somma esente or ulteriore detrazione and applies no substitute tax. Each of these capabilities records a not_withholding_agent decision. See Domestic work.

Family deductions (Art. 12 TUIR)¶

The deductions of art. 12 TUIR (text in force read on Normattiva on 3 October 2026) are computed on the reddito complessivo of the year: the employment income the run projects for the year plus the income of CurrentYearTaxFacts (other employers, other income), less the income of the main dwelling (c. 4-bis). Amounts and limits are data of family/<year>.json, each block with its provenance:

• Truncation (c. 4). Each ratio is taken to four decimals, the rest discarded, before it multiplies the amount; a ratio of zero or less, or of one (no income) for children and ascendants, gives no deduction.
• Months (c. 3). A deduction is due from the month its conditions start to the month they end, both included: Dependent.dependent_from and dependent_until date the dependency (marriage, separation, cohabitation), and a child counts from the month it turns 21 to the month it turns 30. The full-year amount is multiplied by the months over twelve and by allocation_pct, then rounded to the cent once.
• Own income (c. 2). 2,840.51, or 4,000 for a child who turns at most 24 in the year (the whole year, as the Agenzia delle Entrate 730 instructions read "non superiore a ventiquattro anni").
• Sole parent (lett. c, last period). With FamilyComposition.sole_parent the eldest entitled child takes the spouse deduction when it is higher.
• Conditions are stated, never assumed. The engine cannot verify the conditions of a dependant; the caller states them on Dependent, and a condition left None is unknown, not met:

A dependant that may qualify in some month (no stated condition excludes it, a child within its age band) with an unknown condition takes no deduction: the decision is required_fact_missing with inputs["missing_facts"], and an incomplete issue dependent_condition_unknown per field (fact the field name) gives a missing_fact blocker. An unknown condition of a dependant that cannot qualify (a child under 21 all year, an ascendant stated not cohabiting) blocks nothing. The dependency interval has no default: dependent_from=None and dependent_until=None state an open end and must be passed.

The decision family_deductions records the income, the months of each dependent and the amount per relationship. Its reason:

Missing income is never taken as zero: a worker whose only income is this employment states it with CurrentYearTaxFacts.employment_only(tax_year, estimated_on). A TaxYearPlan.current_year replaces the facts of its competence years, so a December paid in January reads the facts of the year it is paid in.

IVS ceiling¶

The massimale IVS caps the IVS contribution base (and the 1% addizionale) of workers without contributions before 1 January 1996 and of those who opted for the contributory system (L. 335/1995 art. 2 c. 18 and art. 1 c. 23). Its value is data of the INPS rules of the year with their provenance (EUR 122,295 for 2026, INPS Circ. 6/2026). The engine derives the eligibility from Employment.contribution_history (ContributionHistory): the date of the first contribution credited to a mandatory pension scheme and the contributory option. The history is the one in force for the run: pre-1996 periods credited on request lift the massimale only from the month after the request (L. 208/2015 art. 1 c. 280) and the option from when it is exercised, so for earlier runs supply the history as it stood then. The engine models no effective date of a change.

Every run whose INPS rules carry a massimale records an ivs_ceiling_eligibility decision:

With required_fact_missing the decision is provisional and lists the employee and employer contributions of both branches in its inputs; the inps_employee and inps_employer decisions are incomplete with no amount; an ivs_ceiling_eligibility_unknown issue names the fact contribution_history, so the result has a missing_fact blocker and is not payable. The breakdown, the ledger and the net carry the uncapped branch as a simulation, never as a payable amount. The massimale runs across all the employers of the year (INPS circ. 237/2016 par. 3.1): import the base of the others as InpsBaseYtd.other_employers with engine.import_opening_balances. The base and the massimale are those of the competence year of the run: a December paid in January counts toward the massimale of its own year (see INPS base by competence). Conditions that lower the reliability of a result are reported in result.issues, never as free text.

Additional 1% IVS¶

D.L. 384/1992 art. 3-ter charges the worker 1% on the pay above the first pensionable band of the year (EUR 56,224 for 2026), within the massimale when it applies. INPS applies it by the mensilizzazione (INPS circ. 6/2026 par. 5, circ. 7/2010 par. 3, msg. 5327/2015 par. 2.1): each month the 1% is charged on the INPS base of the month above the band "rapportato a dodici mesi" (EUR 4,685 for 2026), whatever the base of the year. The runs of one competence month share its threshold; the state keeps the base of the latest month only, so an adjustment run of an earlier month and a later run of the current one each count from zero, a difference the settlement corrects. The rule, with both thresholds as INPS publishes them, is the employee_additional block of the INPS rules of the year.

The runs of competence December, of the month the employment ends and the termination run settle the year (msg. 5327/2015 par. 2.3; circ. 156/2025 par. 5): 1% of the base of the year within the massimale above the annual band, less the 1% already withheld on the year. A December run that follows another one of the same month settles only what the first left. The settlement can be a credit to the worker: the component addizionale_1pct_conguaglio then carries a negative amount, and the employee contributions of the run may be negative by as much. The monthly component is addizionale_1pct.

The 1% withheld is kept per competence year on InpsBaseYtd.additional_ivs. The bases of other employers count toward the band; import them with what those employers withheld, InpsBaseYtd.other_employers_additional_ivs, from their CU. Left None with a base of other employers, a settling run cannot deduct it: it reports the issue other_employers_additional_ivs_unknown (incomplete, fact="other_employers_additional_ivs"), so the result has a missing_fact blocker and is not payable.

Minimum INPS base¶

The base INPS contributions are computed on has two floors (INPS circ. 6/2026 par. 1):

• the pay of the collective agreement (D.L. 338/1989 art. 1 c. 1), read in the comparatively most representative CCNL of the category (L. 549/1995 art. 2 c. 25). The engine computes the pay of the CCNL of the request from its tables, so this floor is the pay chain itself. Whether that CCNL is the most representative of the category is not a fact the engine holds: a run on a CCNL that is not has to be checked outside the engine;
• the minimum daily pay of D.L. 463/1983 art. 7 c. 1, 9.5% of the minimum FPLD pension: EUR 58.13 a day for 2026. A fully paid month of a full-time, monthly-paid worker counts 26 days (six days of the normal week over 52 weeks and 12 months; the 26 days INPS counts in a full month in the Uniemens technical document), EUR 1,511.38. A part-time worker has the hourly minimum of D.Lgs. 81/2015 art. 11 c. 1, EUR 8.72 for a 40-hour week (circ. 6/2026 par. 4, "58,13 euro x 6/40"), times the contracted weekly hours over the same 26 days. A qualifica above EUR 58.13 in Tabella A of allegato 1 keeps its amount (daily_by_category, with no published hourly minimum).

The run that posts the monthly pay of a fully employed month without an unpaid absence or a sick leave raises its INPS base to that minimum (minimum_base_reason raised_to_minimum); the pay itself, the gross and the TFR quota are unchanged, while the contributions, the 0.50% IVS taken from the TFR, the IRPEF taxable and the year-to-date base follow the raised base. Art. 7 c. 5 excludes apprentices and the operai agricoli (apprentice_excluded, category_excluded); domestic work has its own hourly contributions. Whether the EUR 51.70 Tabella A lists for the operai agricoli is a floor is not settled (limitation inps_minimum_base_exempt_category).

The bundle cannot fix the minimum of a partly employed month, of a month with an absence or a sick leave (whose reduced pay circ. 6/2026 par. 1 and the Uniemens element RispettoMinimale exempt), of a run that adds pay to a month another run posted (an extra month, an adjustment, a termination after the regular run) or settles the extra-month ratei at termination, of a part-time week whose hourly minimum is not published (only 40 and, for the Gestione pubblica, 36 hours are), of the Gestione pubblica (no sourced day count of a month), or of an agricultural level whose category is open. When the base of such a run is below the highest minimum its month can have (26 days, or 27 where the day count is not sourced, fewer days of a six-day week in a partly employed span, scaled to the part-time hours), its inps_employee and inps_employer decisions are incomplete with reason minimum_base_undetermined and no amount, an issue inps_minimum_base_undetermined explains it (with fact="category" for the open category), and the result is not payable. A base above that bound needs no day count and is final. The rule is the minimum_base block of the INPS rules of the year.

API reference: CalculationDecision, REGION_CODES, FamilyComposition