Method and sources

Method

For an employee, Indexo follows the path of a payslip: employee social security contributions, work bonus, withholding tax (bedrijfsvoorheffing / précompte professionnel) under the key formula of Annex III to the Royal Decree implementing the Income Tax Code 1992, reductions for dependants, fiscal work bonus and special social security contribution. Each line is rounded to the cent, as on a payslip. The annual view adds holiday pay and the year-end bonus, which are subject to withholding tax on non-recurring pay, then compares the withholding tax deducted over the year with the estimated final tax.

For a self-employed person, Indexo calculates the year’s final social security contributions, which are deductible, then estimates the tax using the 2026 personal income tax scale and the municipal tax rate you enter. The reverse calculation searches, by successive approximations, for the gross pay or the amount invoiced that gives exactly the net you are aiming for. The engine is checked against payslips and examples published by independent sources; these checks are rerun before every parameter update.

Assumptions

Stable salary over the year; a full year with the same employer for holiday pay; a 38-hour week to estimate the hourly wage; municipal tax of 7% unless you enter another rate; for a couple, each spouse is taxed on their own income.

What the calculation does not cover

Other household income, the tax reduction for service vouchers (regional), family allowance supplements and the German-speaking Community’s amounts, public sector pay scales (you enter the gross salary yourself), time credit and thematic leave allowances, tax for cross-border workers who work abroad, the details of running a company (accounting fees, annual contribution, VVPR-bis conditions), and the tax benefits of old mortgage loans (bonus logement, chèque habitat, woonbonus). Public sector holiday pay and end-of-year allowance, sickness and unemployment benefits, the next indexation and the calculation through a company are simplified estimates, flagged as such in the result.

Parameters used and sources

Each parameter first links to its official text: a publication by the administration (NSSO, FPS Finance, ONVA/RJV, FPS Social Security, FPS Employment) or a law published in the Belgian Official Gazette. The ‘Cross-checked with’ column cites the independent professional publications (social secretariats, trade unions, firms) used to check the amounts, especially when the administration has not yet updated its page. When the administration has not published an up-to-date page, the source is a professional publication: it is then flagged as a secondary source, here and in every result.

Update calendar

Here is when the amounts Indexo uses usually change, and what is checked each time. Next full check: early January 2027.

What next?

• Proof of accuracyCases checked to the cent against independent sources.
• Open dataAll parameters, dated and sourced, in JSON.
• What’s changingThe calendar of changes to rules and amounts.