How the New Zealand sole trader calculator is verified

Inputs

Input	Field	Type
Annual profit after expenses (NZD)	`annualProfit`	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)
Income tax	`incomeTax`	`($p := annualProfit; $round($const.r1 * $min([$p, $const.b1]) + $const.r2 * $max([0, $min([$p, $const.b2]) - $const.b1]) + $const.r3 * $max([0, $min([$p, $const.b3]) - $const.b2]) + $const.r4 * $max([0, $min([$p, $const.b4]) - $const.b3]) + $const.r5 * $max([0, $p - $const.b4]), 2))`
ACC earner levy	`accLevy`	`$round($min([annualProfit, $const.accMaxEarnings]) * $const.accRate, 2)`
Net income / year	`netIncome`	`$round($max([0, annualProfit - incomeTax - accLevy]), 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
`b1`	15600
`b2`	53500
`b3`	78100
`b4`	180000
`r1`	0.105
`r2`	0.175
`r3`	0.3
`r4`	0.33
`r5`	0.39
`accRate`	0.0175
`accMaxEarnings`	156641

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 — 85,000 NZD profit — a typical full-time sole trader

given `annualProfit`	85000
expect `incomeTax`	17927.5
expect `accLevy`	1487.5
expect `netIncome`	65585

Case 2 — 30,000 NZD — part-time

given `annualProfit`	30000
expect `incomeTax`	4158
expect `accLevy`	525
expect `netIncome`	25317

Case 3 — 200,000 NZD — above the ACC maximum liable earnings

given `annualProfit`	200000
expect `incomeTax`	57077.5
expect `accLevy`	2741.22
expect `netIncome`	140181.28

Sources

Inland Revenue / business.govt.nz — Tax basics for sole traders · checked 2026-08-22
NZ tax rates and the 2026/27 ACC earner levy (1.75%, maximum liable earnings 156,641 NZD) · 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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