Belgium methodology (2026)
Model
Private sector employee taxed as a single person with no dependants, income year 2026, municipal surcharge at 7% unless you change it.
Source
The brackets and the exempt amount come from the tax administration's own rates page, which publishes income years 2025 and 2026 side by side. The standard professional expenses figures come from the published indexed amounts for assessment year 2027, whose bracket figures match the administration exactly.
Income year 2026 is assessment year 2027
Belgium labels the same tax two ways, and the brackets differ between adjacent years, so the distinction matters. Income year 2026 uses thresholds of 16,720, 29,510 and 51,070 at rates of 25%, 40%, 45% and 50%, where income year 2025 used 16,320, 28,800 and 49,840. The exempt amount is 11,180 for income year 2026 against 10,910 for 2025. A figure labelled only "2026" could be either.
The exempt amount is a reduction
This is the mechanism most descriptions get wrong. The belastingvrije som does not come off income before the brackets apply. It is granted as a tax reduction valued at the lowest bracket rate, so it cuts the bill by 25% of the exempt amount. Treating it as an income deduction overstates the benefit considerably.
Components
ONSS is 13.07% of gross. Standard professional expenses are 30% of income after ONSS, capped at 6,070, with the cap reached at about 20,233 of taxable income. The brackets then apply to what remains, the exempt amount reduction is subtracted, and the municipal surcharge is charged as a percentage of the resulting income tax rather than of income.
Scope and limits
Two exclusions pull in opposite directions and both are worth knowing. The werkbonus and the tax credit for low activity income, worth up to 2,000 and 910 a year, are not modelled, so this engine materially overstates tax for lower earners. The bijzondere bijdrage sociale zekerheid, an extra employee levy, is also excluded, which runs the other way. Increases in the exempt amount for dependants and any regional component beyond the municipal surcharge are out of scope.
A correction worth recording
Our earlier research claimed a competitor's exempt amount of 11,180 was one indexation step stale and should be 11,550. The tax administration publishes 10,910 for income year 2025 and 11,180 for income year 2026, so the competitor was right and the claim was retracted. It was the second time in this project that several agreeing secondary sources produced a confident but wrong conclusion, which is why these pages now cite the administration directly.
Verification
Twenty automated tests cover the published brackets and exempt amount, the deduction caps, the surcharge mechanism and the guard against reintroducing the 2025 figures. The tax administration's own worked example reproduces exactly: 13,212 of gross tax less 2,727.50 of relief giving 10,484.50.