How the Australia gst calculator is verified
Inputs
| Input | Field | Type |
|---|---|---|
| Amount | amount | number |
| Direction | direction | string |
| B2B reverse charge applies | reverseCharge | boolean |
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 | $const.standardRate |
| Effective Rate | effectiveRate | reverseCharge ? 0 : rate |
| Net Amount | netAmount | direction = 'remove' ? $round(amount / (1 + effectiveRate), 2) : $round(amount, 2) |
| Vat Amount | vatAmount | $round(netAmount * effectiveRate, 2) |
| Gross Amount | grossAmount | $round(netAmount + vatAmount, 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 | 0.1 |
Test vectors (4)
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 — S1 100.00 AUD net at 10%
given amount | 100 |
|---|---|
given direction | add |
given reverseCharge | no |
expect vatAmount | 10 |
expect grossAmount | 110 |
expect netAmount | 100 |
Case 2 — S2 110.00 AUD gross back to net: GST is one eleventh
given amount | 110 |
|---|---|
given direction | remove |
given reverseCharge | no |
expect netAmount | 100 |
expect vatAmount | 10 |
Case 3 — S3 reverse charge: the supplier charges no GST
given amount | 100 |
|---|---|
given direction | add |
given reverseCharge | yes |
expect vatAmount | 0 |
expect grossAmount | 100 |
Case 4 — S4 a 2 200.00 AUD invoice holds 200.00 of GST
given amount | 2200 |
|---|---|
given direction | remove |
given reverseCharge | no |
expect netAmount | 2000 |
expect vatAmount | 200 |
Sources
- Federal Register of Legislation - GST Act 1999 s 9-70 (10%) · checked 2026-10-05
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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