How the Belgium netto loon calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| Bruto jaarloon (EUR) | grossAnnual | number |
The formula
Each derived value below is computed by the Valem engine from the expression shown, in this order. Nothing else runs — there is no hidden code path.
| Derived value | Field | Formula (JSONata) |
|---|---|---|
| RSZ (13,07%) | rsz | $round(grossAnnual * $const.rszRate, 2) |
| Beroepskosten | profCosts | ($p := (grossAnnual - rsz) * 0.30; $round($p < $const.profCostCap ? $p : $const.profCostCap, 2)) |
| Belastbaar | taxable | $round(grossAnnual - rsz - profCosts, 2) |
| Belasting (incl. gemeente) | tax | ($t := taxable; $g := (($t < 16320 ? $t : 16320) * 0.25) + (($t > 16320 ? (($t < 28800 ? $t : 28800) - 16320) : 0) * 0.40) + (($t > 28800 ? (($t < 49840 ? $t : 49840) - 28800) : 0) * 0.45) + (($t > 49840 ? ($t - 49840) : 0) * 0.50); $b := $g - $const.taxFreeCredit; $b := $b > 0 ? $b : 0; $round($b * (1 + $const.municipalRate), 2)) |
| Netto / jaar | netAnnual | $round(grossAnnual - rsz - tax, 2) |
| Netto / maand | netMonthly | $round(netAnnual / 12, 2) |
Rates and thresholds
The expressions above read these as $const. They are the figures the annual refresh replaces, so they live as data rather than being written into the formulas — which is what makes a year-on-year change a one-line diff instead of an edit to arithmetic.
| Constant | Value |
|---|---|
rszRate | 0.1307 |
profCostCap | 5930 |
taxFreeCredit | 2727.5 |
municipalRate | 0.07 |
Test vectors (2)
Every case is executed against this model on each build, by the Java engine and independently by a JavaScript one. A mismatch of a single cent fails the build, so the page cannot ship advertising a figure the model no longer produces.
Case 1 — S1 45,000 EUR
given grossAnnual | 45000 |
|---|---|
expect rsz | 5881.5 |
expect profCosts | 5930 |
expect taxable | 33188.5 |
Case 2 — S3 90,000 EUR
given grossAnnual | 90000 |
|---|---|
expect rsz | 11763 |
expect taxable | 72307 |
Sources
- FOD Financiën Tax-Calc · checked 2026-08-02
- RSZ · checked 2026-08-02
Check it yourself — and reuse it
The whole model as JSON — inputs, outputs, formulas, vectors and sources in one re-runnable document.
This model is published under CC BY 4.0: use it, adapt it, build on it, with attribution. The licence covers the model — the expression of each rule and the arrangement of the catalog. The rates and thresholds are law, and nobody licenses those; the sources above are where they come from.
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