How the Iceland vsk (vat) calculator is verified

Inputs

Input	Field	Type
Amount (ISK)	`amount`	number
Direction	`direction`	string
What is being supplied	`category`	string
No VAT charged by you: an export, or the customer accounts for the tax	`reverseCharge`	boolean
Taxable sales in the last twelve months (ISK)	`turnover12Months`	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 for the category	`rate`	`category = 'reduced' ? $const.reducedRate : $const.standardRate`
Rate applied	`effectiveRate`	`reverseCharge ? 0 : rate`
Net	`netAmount`	`direction = 'remove' ? $round(amount / (1 + effectiveRate), 2) : $round(amount, 2)`
VAT	`vatAmount`	`$round(netAmount * effectiveRate, 2)`
Gross	`grossAmount`	`$round(netAmount + vatAmount, 2)`
Must register for VAT	`mustRegister`	`turnover12Months > $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`	0.24
`reducedRate`	0.11
`registrationThreshold`	2000000

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 10,000 ISK net at the general rate

given `amount`	10000
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `turnover12Months`	0
expect `vatAmount`	2400
expect `grossAmount`	12400
expect `netAmount`	10000

Case 2 — S2 12,400 ISK gross back to net at the general rate

given `amount`	12400
given `direction`	remove
given `category`	standard
given `reverseCharge`	no
given `turnover12Months`	0
expect `netAmount`	10000
expect `vatAmount`	2400

Case 3 — S3 food at 11%

given `amount`	10000
given `direction`	add
given `category`	reduced
given `reverseCharge`	no
given `turnover12Months`	0
expect `vatAmount`	1100
expect `grossAmount`	11100

Case 4 — S4 Skatturinn’s share of a VAT-inclusive price: 19.35% at 24%

given `amount`	10000
given `direction`	remove
given `category`	standard
given `reverseCharge`	no
given `turnover12Months`	0
expect `netAmount`	8064.52
expect `vatAmount`	1935.48

Case 5 — S5 exactly ISK 2,000,000 does not yet require registration

given `amount`	0
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `turnover12Months`	2000000
expect `mustRegister`	false

Case 6 — S6 above ISK 2,000,000 it does

given `amount`	0
given `direction`	add
given `category`	standard
given `reverseCharge`	no
given `turnover12Months`	2000001
expect `mustRegister`	true

Sources

Skatturinn — Skattskylda og skattprósentur: the two VAT rates, 24% and 11%, and the 11% categories (lög nr. 50/1988) · checked 2026-10-12
Skatturinn — Skráning á VSK-skrá: registration once taxable sales exceed ISK 2,000,000 in any twelve-month period · checked 2026-10-12

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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