All four figures are from the year before last: for the 2026 instalments, they are the 2024 ones. They all appear on that year's assessment notice. The assessed tax is the figure left after tax credits, except the disability credit.

Only part of your assessed tax enters this calculation. Because category B accounts for this percentage of your net income, that is the share of the assessment the formula uses. Someone with only category-B income puts the whole assessment in, which is why calculations that apply the percentage to the full amount produce a higher figure.

The rate in force is 65%. Up to the 2024 instalments article 102 read 76.5%, and the Lei n.º 45-A/2024 of 31 December cut it; the 2026 State Budget kept it at 65%. Because most articles published on the subject predate 2025, they still explain the old rule.

The calculator applies the formula in article 102 of the Portuguese Income Tax Code and does not replace the assessment notice, which is where the tax authority states the official amount of each instalment. Out of scope: working out the assessed tax and its credits, determining net category-B income, corporate payments on account, and the exact compensatory interest of paragraph 6, which depends on a day count.

Educational estimate based on the figures you enter. It is not tax advice.