Employee (contrat de travail / arbeidsovereenkomst)

• Employee social security (RSZ/ONSS): 13.07% of gross pay, no cap. We model a white-collar employee (base = 100% of gross); blue-collar workers contribute on 108% of gross.
• Forfaitary professional expenses: 30% of gross pay after ONSS, capped at EUR 6,070 for income year 2026.
• Personal income tax (IPP/PB), income year 2026 brackets on annual taxable income: 25% up to EUR 16,720, 40% to EUR 29,510, 45% to EUR 51,070, 50% above.
• Tax-free sum (belastingvrije som / quotite exemptee): EUR 11,550 for income year 2026 under the December 2025 personal income tax reform. The relief equals the bracket tax on that amount (EUR 2,887.50) and cannot push the tax below zero. PROVISIONAL: the indexation-only amount under the previous law was EUR 11,180; verify against the final published tables.
• Communal surcharge (centimes additionnels / aanvullende gemeentebelasting): a commune-set percentage of the state income tax. Default 7% in this calculator, adjustable; a few communes charge 0% and some charge up to around 9%.
• Monthly figures are annual tax divided by 12. Real payroll withholding uses separate scales and can differ slightly from the final assessment.
• Employer cost: the basic employer ONSS rate in the private profit sector is 25%; sector funds, insurances and the structural reduction move the real all-in figure. We use an approximate, adjustable 27% all-in default.
• Out of scope: the werkbonus / bonus a l'emploi reduction for low salaries (social and fiscal; nets at low salaries are understated here - PROVISIONAL), exact withholding scales, marriage quotient and dependants, 13th month and double holiday pay, regional differences, the special social security contribution, benefits in kind, actual professional expenses.

Sources: FPS Finance - Tax rates, FPS Finance - Professional income, Social Security (RSZ/ONSS) - Contributions.

Self-employed (independant / zelfstandige, main occupation)

• Profit = annual revenue minus annual business expenses (actual expenses; the optional legal expense forfait for profits is out of scope).
• Social contributions (INASTI/RSVZ statute), contribution year 2026: 20.50% of the net taxable income up to EUR 75,024.54, then 14.16% on the part up to the absolute ceiling of EUR 110,562.42, nothing above. Minimum base for a main occupation: EUR 17,374.08 (minimum quarterly contribution EUR 890.42 before fund fees).
• Contributions are deductible from their own base, so the calculator solves the resulting circular equation numerically.
• Social insurance fund management fees: a percentage of the contributions, set by each fund (roughly 3.05% to 4.25%). Default 3.05%, adjustable; also deductible.
• Income tax: the same income year 2026 brackets, tax-free sum and communal surcharge as for employees, applied to profit minus contributions and fees. No 30% expense forfait here because actual expenses are an input.
• Out of scope: starter (primostarter) reduced minimum contributions, company director regimes, VAT, advance tax payments (versements anticipes / voorafbetalingen) bonuses and surcharges, pension savings (PLCI/VAPZ), the quarterly minimum at zero income.

Secondary occupation (independant complementaire / zelfstandige in bijberoep)

• For someone who is self-employed on the side of a main job (at least a half-time employment or equivalent), the INASTI schedule keeps the same 20.50% and 14.16% tiers and ceilings but drops the main-occupation minimum base.
• Exemption floor: if the annual net taxable income from the side activity stays below EUR 1,922.16 (2026; 2025: EUR 1,865.45), no social contributions are due at all.
• At or above the floor, contributions are 20.50% of the actual income, with a small minimum of EUR 98.51 per quarter (EUR 394.04 per year, which is 20.50% of the floor) before fund fees. Contributions and fund fees stay deductible from the taxable base.
• Simplification: the calculator tests the exemption floor against the profit before deducting the contributions themselves; INASTI assesses it on the final net taxable income, which can differ slightly in a narrow band around the floor.
• Income tax on the side profit: the main salary is not modelled here, but in the real assessment the side profit is added on top of it. The lower brackets and the tax-free sum are therefore already used up by the salary, and the side profit is effectively taxed at the person's marginal rate. The calculator applies a flat marginal rate to the side taxable income (adjustable, default 50% - the top bracket, reached above EUR 51,070 of taxable income), plus the communal surcharge on that tax. Set it lower (e.g. 40% or 45%) if the combined income stays in a lower bracket.
• This means the result shows only the net from the side activity, not a full household net; the main job's payslip is unchanged by this calculation.
• Out of scope for the secondary occupation: the article 37 assimilation cases (low-income main-occupation treatment), student-entrepreneur and pensioner schedules, the impact of the side income on tax credits tied to the main job.

Sources: INASTI/RSVZ - Social contributions, INASTI/RSVZ - Secondary occupation, INASTI/RSVZ - Figures, amounts and limits, Liantis - Contributions in secondary occupation (2026 table), FPS Finance - Tax rates.