How the Japan consumption tax calculator is verified

Inputs

Input	Field	Type
Amount	`amount`	number
Direction	`direction`	string
What is being supplied	`category`	string
Taxable sales in the base period	`annualTurnover`	number
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 = 'reduced1' ? $const.reducedRate1 : $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)`
Must Register	`mustRegister`	`annualTurnover > $const.registrationThreshold`

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
`reducedRate1`	0.08
`registrationThreshold`	10000000

Test vectors (6)

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 JPY net at the standard rate

given `amount`	1000
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	0
expect `vatAmount`	100
expect `grossAmount`	1100
expect `netAmount`	1000

Case 2 — S2 1100 JPY gross back to net at 10%

given `amount`	1100
given `direction`	remove
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	0
expect `netAmount`	1000
expect `vatAmount`	100

Case 3 — S3 1000 JPY of groceries at the 8% reduced rate

given `amount`	1000
given `direction`	add
given `category`	reduced1
given `reverseCharge`	no
given `annualTurnover`	0
expect `vatAmount`	80
expect `grossAmount`	1080

Case 4 — S4 reverse charge: the supplier charges no tax

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

Case 5 — S5 base-period sales above 10 million yen: taxable enterprise

given `amount`	100
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	12000000
expect `mustRegister`	true

Case 6 — S6 base-period sales of exactly 10 million yen: exempt (the statute says "equal to or less than")

given `amount`	100
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	10000000
expect `mustRegister`	false

Sources

National Tax Agency - Consumption Tax: 10% standard, 8% reduced · checked 2026-10-05
National Tax Agency - exemption where base-period taxable sales are 10 million yen or less · 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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