How the Singapore road tax calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| Engine capacity (cc) | cc | number |
| Vehicle age (years) | ageYears | 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) |
|---|---|---|
| Base 6-monthly road tax | baseSixMonth | ($c := cc; $b := $c <= 600 ? 200 : $c <= 1000 ? 200 + 0.125 * ($c - 600) : $c <= 1600 ? 250 + 0.375 * ($c - 1000) : $c <= 3000 ? 475 + 0.75 * ($c - 1600) : 1525 + 1.0 * ($c - 3000); $round($b * $const.rebateFactor, 2)) |
| Age surcharge | ageSurcharge | ($a := ageYears; $a < 10 ? 0 : $a < 11 ? 0.10 : $a < 12 ? 0.20 : $a < 13 ? 0.30 : $a < 14 ? 0.40 : 0.50) |
| 6-monthly payable | sixMonth | $round(baseSixMonth * (1 + ageSurcharge), 2) |
| Annual road tax | tax | $round(sixMonth * 2, 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 |
|---|---|
rebateFactor | 0.782 |
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 — C1 1,598 cc sedan, 3 years old — the worked LTA example
given cc | 1598 |
|---|---|
given ageYears | 3 |
expect baseSixMonth | 370.86 |
expect ageSurcharge | 0 |
expect sixMonth | 370.86 |
expect tax | 741.72 |
Case 2 — C2 same car at 12 years — 30% age surcharge
given cc | 1598 |
|---|---|
given ageYears | 12 |
expect ageSurcharge | 0.3 |
expect sixMonth | 482.12 |
expect tax | 964.24 |
Case 3 — C3 small 600 cc car — the flat bottom band
given cc | 600 |
|---|---|
given ageYears | 2 |
expect baseSixMonth | 156.4 |
expect tax | 312.8 |
Sources
- OneMotoring road tax calculator · checked 2026-08-05
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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