How the Japan take-home pay calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| 年収 (JPY) | 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) |
|---|---|---|
| 社会保険 (~14.66%) | social | $round(grossAnnual * $const.socialRate, 2) |
| Taxable | taxable | ($b := grossAnnual - social - $const.deduction; $round($b > 0 ? $b : 0, 2)) |
| 所得税 | nationalTax | ($t := taxable; $round((($t < 1950000 ? $t : 1950000) * 0.05) + (($t > 1950000 ? (($t < 3300000 ? $t : 3300000) - 1950000) : 0) * 0.10) + (($t > 3300000 ? (($t < 6950000 ? $t : 6950000) - 3300000) : 0) * 0.20) + (($t > 6950000 ? (($t < 9000000 ? $t : 9000000) - 6950000) : 0) * 0.23) + (($t > 9000000 ? (($t < 18000000 ? $t : 18000000) - 9000000) : 0) * 0.33) + (($t > 18000000 ? ($t - 18000000) : 0) * 0.40), 2)) |
| 住民税 (10%) | residentTax | $round(taxable * $const.residentRate, 2) |
| 手取り / 年 | netAnnual | $round(grossAnnual - social - nationalTax - residentTax, 2) |
| 手取り / 月 | 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 |
|---|---|
socialRate | 0.1469 |
deduction | 1000000 |
residentRate | 0.1 |
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 5,000,000 JPY
given grossAnnual | 5000000 |
|---|---|
expect social | 734500 |
Case 2 — S2 7,000,000 JPY
given grossAnnual | 7000000 |
|---|---|
expect social | 1028300 |
Sources
- 国税庁 (NTA) · 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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