How the Japan 個人事業主 calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| Annual business income after expenses (JPY) | annualBusinessIncome | 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) |
|---|---|---|
| National pension | nationalPension | $round($const.pensionMonthly * 12, 2) |
| After Blue Return | afterBlueReturn | $round($max([0, annualBusinessIncome - $const.blueReturnDeduction]), 2) |
| Taxable Income | taxableIncome | $round($max([0, afterBlueReturn - $const.basicDeduction - nationalPension]), 2) |
| Income tax (incl. surtax) | incomeTax | ($t := taxableIncome; $round(($const.r1 * $min([$t, $const.b1]) + $const.r2 * $max([0, $min([$t, $const.b2]) - $const.b1]) + $const.r3 * $max([0, $min([$t, $const.b3]) - $const.b2]) + $const.r4 * $max([0, $min([$t, $const.b4]) - $const.b3]) + $const.r5 * $max([0, $min([$t, $const.b5]) - $const.b4]) + $const.r6 * $max([0, $min([$t, $const.b6]) - $const.b5]) + $const.r7 * $max([0, $t - $const.b6])) * (1 + $const.reconstructionRate), 2)) |
| Resident tax (10%) | residentTax | $round($const.residentRate * $max([0, afterBlueReturn - $const.residentBasicDeduction - nationalPension]), 2) |
| Net income / year | netIncome | $round($max([0, annualBusinessIncome - nationalPension - incomeTax - residentTax]), 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 |
|---|---|
blueReturnDeduction | 650000 |
pensionMonthly | 17920 |
basicDeduction | 580000 |
residentBasicDeduction | 430000 |
residentRate | 0.1 |
reconstructionRate | 0.021 |
b1 | 1950000 |
b2 | 3300000 |
b3 | 6950000 |
b4 | 9000000 |
b5 | 18000000 |
b6 | 40000000 |
r1 | 0.05 |
r2 | 0.1 |
r3 | 0.2 |
r4 | 0.23 |
r5 | 0.33 |
r6 | 0.4 |
r7 | 0.45 |
Test vectors (3)
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 — 6,000,000 JPY business income — a typical full-time sole proprietor
given annualBusinessIncome | 6000000 |
|---|---|
expect nationalPension | 215040 |
expect afterBlueReturn | 5350000 |
expect taxableIncome | 4554960 |
expect incomeTax | 493645.33 |
expect residentTax | 470496 |
expect netIncome | 4820818.67 |
Case 2 — 2,500,000 JPY — part-time
given annualBusinessIncome | 2500000 |
|---|---|
expect nationalPension | 215040 |
expect afterBlueReturn | 1850000 |
expect taxableIncome | 1054960 |
expect incomeTax | 53855.71 |
expect residentTax | 120496 |
expect netIncome | 2110608.29 |
Case 3 — 15,000,000 JPY — into the 33% band
given annualBusinessIncome | 15000000 |
|---|---|
expect nationalPension | 215040 |
expect afterBlueReturn | 14350000 |
expect taxableIncome | 13554960 |
expect incomeTax | 2998816.67 |
expect residentTax | 1370496 |
expect netIncome | 10415647.33 |
Sources
- 国税庁 — 所得税の税率および青色申告特別控除 · checked 2026-08-22
- 2026年度（令和8年度）国民年金保険料 月額17,920円 · checked 2026-08-22
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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