How the India road tax calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| Ex-showroom price (INR) | exShowroom | number |
| State of registration | state | string |
| Rate if "Other" (fraction, e.g. 0.10) | customRate | 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) |
|---|---|---|
| Rate | rate | ($s := state; $s = 'KA' ? $const.rateKA : $s = 'DL' ? $const.rateDL : $s = 'TN' ? $const.rateTN : $s = 'MH' ? $const.rateMH : customRate) |
| One-time road tax | roadTax | $round(exShowroom * rate, 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 |
|---|---|
rateKA | 0.18 |
rateDL | 0.07 |
rateTN | 0.13 |
rateMH | 0.11 |
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 Karnataka, ex-showroom ₹1,200,000 at 18%
given exShowroom | 1200000 |
|---|---|
given state | KA |
expect roadTax | 216000 |
Case 2 — C2 Delhi, ex-showroom ₹800,000 at 7%
given exShowroom | 800000 |
|---|---|
given state | DL |
expect roadTax | 56000 |
Case 3 — C3 Tamil Nadu, ex-showroom 1,000,000 at 13%
given exShowroom | 1000000 |
|---|---|
given state | TN |
expect rate | 0.13 |
expect roadTax | 130000 |
Sources
- Vahan — Ministry of Road Transport · 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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