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What to confirm before calculating VAT
Check whether the amount is a total, a supply amount, or a tax amount before using the calculator.
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A total amount is not the same as a supply amount
A total amount includes both the supply amount and VAT, so the starting number must be clear before calculation begins.
If you mix up gross and supply values, the result changes immediately even when the tax rate stays the same.
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Confirm tax treatment first
The usual VAT rate may be 10 percent, but exempt or zero-rated transactions need different handling.
The same type of transaction can also be treated differently depending on business type and invoicing rules.
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Rounding rules can change the result
VAT amounts can differ by a small margin depending on whether amounts are rounded or truncated.
It is safer to match your trading partner or accounting standard before finalizing a value.