How the Netherlands btw (vat) calculator is verified

Inputs

Input	Field	Type
Amount	`amount`	number
Direction	`direction`	string
What is being supplied	`category`	string
Turnover this calendar year	`annualTurnover`	number
B2B reverse charge applies	`reverseCharge`	boolean
Turnover last year	`priorYearTurnover`	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`	`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`	`priorYearTurnover > $const.registrationThreshold or 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`	0.21
`reducedRate1`	0.09
`registrationThreshold`	20000

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 100.00 EUR net at the algemeen tarief

given `amount`	100
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	0
given `priorYearTurnover`	0
expect `vatAmount`	21
expect `grossAmount`	121
expect `netAmount`	100

Case 2 — S2 121.00 EUR gross back to net at 21%

given `amount`	121
given `direction`	remove
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	0
given `priorYearTurnover`	0
expect `netAmount`	100
expect `vatAmount`	21

Case 3 — S3 100.00 EUR net at the 9% verlaagd tarief

given `amount`	100
given `direction`	add
given `category`	reduced1
given `reverseCharge`	no
given `annualTurnover`	0
given `priorYearTurnover`	0
expect `vatAmount`	9
expect `grossAmount`	109

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

given `amount`	100
given `direction`	add
given `category`	standard
given `reverseCharge`	yes
given `annualTurnover`	0
given `priorYearTurnover`	0
expect `vatAmount`	0
expect `grossAmount`	100

Case 5 — S5 either year over 20 000 EUR: the KOR is not available

given `amount`	100
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	5000
given `priorYearTurnover`	25000
expect `mustRegister`	true

Case 6 — S6 both years under the limit: the KOR is available

given `amount`	100
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `annualTurnover`	15000
given `priorYearTurnover`	15000
expect `mustRegister`	false

Sources

Belastingdienst - Btw-tarieven en vrijstellingen (21%, 9%) · checked 2026-10-05
Belastingdienst - Voorwaarden voor de kleineondernemersregeling (KOR), EUR 20.000 · 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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