How the United Kingdom vehicle tax calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| CO₂ emissions (g/km) | co2 | number |
| List price (GBP) | listPrice | 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) |
|---|---|---|
| First-year rate | firstYearVED | ($c := co2; $c = 0 ? 10 : $c <= 50 ? 115 : $c <= 75 ? 135 : $c <= 90 ? 280 : $c <= 100 ? 365 : $c <= 110 ? 405 : $c <= 130 ? 455 : $c <= 150 ? 560 : $c <= 170 ? 1410 : $c <= 190 ? 2270 : $c <= 225 ? 3420 : $c <= 255 ? 4850 : 5690) |
| Expensive-car supplement | expensiveSupplement | listPrice > $const.expensiveThreshold ? $const.expensiveSupplementRate : 0 |
| Standard rate (year 2+) | standardVED | $round($const.standardRate + expensiveSupplement, 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 |
|---|---|
standardRate | 200 |
expensiveThreshold | 40000 |
expensiveSupplementRate | 440 |
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 — C1 petrol 128 g/km, list £30,000
given co2 | 128 |
|---|---|
given listPrice | 30000 |
expect firstYearVED | 455 |
expect expensiveSupplement | 0 |
expect standardVED | 200 |
Case 2 — C2 190 g/km, list £42,000 — expensive-car supplement
given co2 | 190 |
|---|---|
given listPrice | 42000 |
expect firstYearVED | 2270 |
expect expensiveSupplement | 440 |
expect standardVED | 640 |
Sources
- GOV.UK — Vehicle tax rate tables · 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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