How the India gst calculator is verified

Inputs

Input	Field	Type
Amount	`amount`	number
Direction	`direction`	string
Which rate schedule applies	`category`	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`	`category = 'scheduleI' ? $const.scheduleI : category = 'scheduleIII' ? $const.scheduleIII : category = 'scheduleIV' ? $const.scheduleIV : category = 'scheduleV' ? $const.scheduleV : category = 'scheduleVI' ? $const.scheduleVI : category = 'scheduleVII' ? $const.scheduleVII : $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`	0.18
`scheduleI`	0.05
`scheduleIII`	0.4
`scheduleIV`	0.03
`scheduleV`	0.0025
`scheduleVI`	0.015
`scheduleVII`	0.28

Test vectors (9)

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 1000.00 INR at the general 18% (Schedule II)

given `amount`	1000
given `direction`	add
given `category`	standard
given `reverseCharge`	no
expect `vatAmount`	180
expect `grossAmount`	1180
expect `netAmount`	1000

Case 2 — S2 1180.00 INR gross back to net at 18%

given `amount`	1180
given `direction`	remove
given `category`	standard
given `reverseCharge`	no
expect `netAmount`	1000
expect `vatAmount`	180

Case 3 — S3 1000.00 INR at 5% (Schedule I)

given `amount`	1000
given `direction`	add
given `category`	scheduleI
given `reverseCharge`	no
expect `vatAmount`	50
expect `grossAmount`	1050

Case 4 — S4 28% survives the 2025 reform (Schedule VII)

given `amount`	1000
given `direction`	add
given `category`	scheduleVII
given `reverseCharge`	no
expect `rate`	0.28
expect `vatAmount`	280

Case 5 — S5 1000.00 INR of demerit goods at 40% (Schedule III)

given `amount`	1000
given `direction`	add
given `category`	scheduleIII
given `reverseCharge`	no
expect `vatAmount`	400
expect `grossAmount`	1400

Case 6 — S6 100000.00 INR of gold at 3% (Schedule IV)

given `amount`	100000
given `direction`	add
given `category`	scheduleIV
given `reverseCharge`	no
expect `vatAmount`	3000

Case 7 — S7 100000.00 INR of rough diamonds at 0.25% (Schedule V)

given `amount`	100000
given `direction`	add
given `category`	scheduleV
given `reverseCharge`	no
expect `rate`	0.0025
expect `vatAmount`	250

Case 8 — S8 100000.00 INR at 1.5% (Schedule VI)

given `amount`	100000
given `direction`	add
given `category`	scheduleVI
given `reverseCharge`	no
expect `vatAmount`	1500

Case 9 — S9 reverse charge: the supplier charges no GST

given `amount`	1000
given `direction`	add
given `category`	standard
given `reverseCharge`	yes
expect `vatAmount`	0
expect `grossAmount`	1000

Sources

CBIC - Notification No. 9/2025-Integrated Tax (Rate), 17.09.2025: all seven rate schedules · 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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