📌 Key Takeaways
- ✓Never subtract 20% from a gross price — £120 minus 20% is £96, but the true net is £100.
- ✓To extract 20% VAT, divide the gross price by 1.20 to find the net amount, or multiply by the statutory VAT fraction 1/6 (approx. 16.6667%).
- ✓To extract 5% reduced VAT, divide gross by 1.05 or multiply by the VAT fraction 1/21.
- ✓HMRC rules require commercial half-up rounding on individual line items or total invoice VAT.
- ✓UtilityFinch VAT calculator performs this exact decimal math with zero floating-point artifacts.
1. The Common 20% Deduction Mistake
When an invoice or till receipt shows a gross total of £120.00 including 20% VAT, many people instinctively calculate 20% of £120 (£24.00) and subtract it, arriving at £96.00 net.
This is a major mathematical error. The 20% VAT was originally added onto the net price, not the gross price:
Net (£100.00) + 20% VAT (£20.00) = Gross (£120.00). The actual VAT portion is £20.00, not £24.00. Deducting 20% from the gross amount overstates the VAT by £4.00!
Over-reporting or under-reporting input VAT due to improper backwards calculations leads to errors on your quarterly VAT return that HMRC may audit.
2. The Statutory HMRC VAT Fractions
HMRC explicitly defines statutory VAT fractions in VAT Notice 700 to extract VAT from gross sums:
For standard rate 20% VAT: The fraction is 1/6 (or 20 ÷ 120). Multiplying any gross figure by 1/6 gives the exact VAT amount.
For reduced rate 5% VAT: The fraction is 1/21 (or 5 ÷ 105). Multiplying a reduced gross figure by 1/21 extracts the 5% VAT portion.
| VAT Rate | Gross Multiplier (Net) | VAT Fraction (Tax) | Worked Example (£120 Gross) |
|---|---|---|---|
| 20% Standard | Gross ÷ 1.20 | Gross × (1/6) | Net: £100.00 | VAT: £20.00 |
| 5% Reduced | Gross ÷ 1.05 | Gross × (1/21) | Net: £114.29 | VAT: £5.71 |
| 0% Zero Rate | Gross ÷ 1.00 | 0 | Net: £120.00 | VAT: £0.00 |
3. HMRC Official Rounding Rules
Under HMRC guidelines, when calculating VAT on invoices or till transactions, you may round down to the nearest whole penny, or round to the nearest penny using commercial rounding (half-up).
However, whatever rounding convention you choose must be applied consistently across your sales and purchase ledgers.
Our online UK VAT Calculator lets you switch between adding VAT and extracting VAT backwards with one click, showing transparent arithmetic steps.
UK VAT Calculator
Calculate VAT additions, reversals, net-to-gross, and gross-to-net at standard (20%), reduced (5%), and zero rates.