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Methodology

The calculator uses straightforward VAT arithmetic. It keeps the underlying calculation at normal JavaScript numeric precision and rounds monetary figures to two decimal places for display.

Adding VAT

For a net amount, the gross amount is calculated as net × (1 + rate ÷ 100). VAT is net × (rate ÷ 100). At 20%, £100 net becomes £120 gross and contains £20 VAT.

Removing VAT

For a gross amount, the net amount is calculated as gross ÷ (1 + rate ÷ 100). VAT is the difference between gross and net. At 20%, £120 gross becomes £100 net and contains £20 VAT.

Rates supported

The default presets are the current UK standard rate of 20%, reduced rate of 5% and zero rate of 0%. A custom rate is available as a secondary option for users who need to check a different percentage. A custom percentage does not mean that rate is valid for a particular UK supply.

Rounding

Results are calculated before display rounding and then shown to the nearest penny. This is a single-amount calculator; it does not model every invoice-line or VAT-return rounding method that may apply in bookkeeping systems.

Official sources

Rates were last checked 10 September 2026 against GOV.UK VAT rates. The £90,000 registration threshold and its rolling-12-month / next-30-days tests were checked against GOV.UK registration guidance. The distinction between zero-rated and exempt supplies is based on HMRC exemption guidance.

What this calculator does not cover

It does not calculate VAT returns, the Flat Rate Scheme, margin schemes, partial exemption, import VAT, EU VAT rates or VAT reverse-charge accounting. Those areas can require additional facts and rules that do not belong in a quick add/remove VAT tool.