One avo is earned per month with at least 15 days worked; each avo is worth 1/12 of the salary. Months with fewer than 15 days do not count.
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Calculate proportional 13th salary with INSS and exclusive-source IRRF.
The 13th salary (Law 4.090/1962) is a bonus equal to one month's salary per year, paid proportionally to time worked. The math revolves around "avos": each month with at least 15 days worked is worth 1/12 of the salary. It is paid in two installments, the 1st, between February and November 30, is 50% with no deductions; the 2nd, by December 20, carries the deductions. And there is an important tax quirk: INSS accrues on the full 13th amount (computed apart from the month's payroll) and IRRF is withheld by exclusive-source taxation, separate from the regular salary. This calculator applies all of that, including averages of habitual variable pay, as an estimate.
The 13th is proportional to the avos. Someone who worked the whole year has 12 avos (a full 13th); someone hired in April, say, and working through December has 9 avos (9/12 of the salary). The 15-day rule is the dividing line: a month counts as an avo only if it had at least 15 days worked.
Habitual variable pay enters the base: overtime, commissions and habitual additionals are included by their average over the year, not just the fixed salary. That is why someone who works a lot of overtime usually has a 13th larger than the standalone base salary.
Suppose a gross 13th of R$ 3,000 (full salary). The 1st installment is 50% = R$ 1,500, paid with no deductions by November. The 2nd installment starts from the other R$ 1,500, but the deductions come out of it: INSS is computed on the full amount (R$ 3,000, by the progressive table) and IRRF by exclusive-source taxation.
That is why the 2nd installment comes in smaller than the 1st: it concentrates all the deductions of the 13th. The tool shows the proportional gross, each installment, INSS, IRRF and the net total, so you can check against the December pay stub.
The 13th does not add to the December salary for tax purposes: it is taxed exclusively at source. In practice, the progressive IRRF table is applied directly to the 13th's base (after INSS and per-dependent deductions), separate from the monthly income, which avoids pushing the month's salary into a higher bracket.
INSS is also computed apart: it accrues once on the full 13th amount, with the progressive table and respecting the contribution ceiling, not on each installment separately. That separation is what makes the 13th's calculation different from ordinary salary.
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></iframe>These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
One avo is earned per month with at least 15 days worked; each avo is worth 1/12 of the salary. Months with fewer than 15 days do not count.
⚠️ Educational estimate. Confirm with accountant or payroll professional.
Lei 4.090/1962 + Lei 4.749/1965; INSS sobre total do 13º (tributação separada); IRRF exclusivo na fonte; avos por ≥15 dias trabalhados/mês.