If you run a small business in the UK, you've probably hit this moment: a client pays you £1,200, the invoice says "VAT included", and you need to work out how much of that is actually VAT and how much is your sale price. The trouble is that you can't just take 20% off the total — that gives the wrong answer. You need a reverse VAT calculation.
Why "just take 20% off" is wrong
The standard UK VAT rate is 20%, but that 20% applies to the net (pre-VAT) price, not the gross amount. So on a £100 net sale, VAT is £20 and the gross is £120. Go backwards from £120 and divide by 5? £120 × 0.20 = £24 — which is too much. The correct VAT inside £120 is £20.
The correct formula is: VAT = gross price × (rate ÷ (100 + rate)). At the 20% standard rate, that's gross × (20/120), or gross ÷ 6. So £120 ÷ 6 = £20 VAT, £100 net. For the 5% reduced rate, divide the gross by 21; for the 0% rate, obviously, the VAT is £0.
The practical shortcut: at 20%, divide the gross price by 6 to get the VAT element, then subtract to get the net. On £1,200 gross: £1,200 ÷ 6 = £200 VAT, £1,000 net. Memorise "divide by 6" and you'll never need to guess again.
When freelancers actually need this
Reverse VAT calculation comes up more often than most people expect:
- Quoting when prices must be VAT-inclusive. If you advertise a flat fee that includes VAT, you need to know your real take-home after stripping the VAT back out.
- Checking a supplier invoice. When a supplier quotes "£600 including VAT", the reverse calculation tells you the net amount you can claim back on your VAT return (assuming the supplier is VAT-registered and the supply is standard-rated).
- Filling in your VAT return. Box 1 (output VAT due) has to reflect the VAT element of your VAT-inclusive sales — not 20% of the gross. Getting this wrong is one of the most common mistakes on first VAT returns, and it either costs you money or leaves you short with HMRC.
- Flat-rate scheme users. If you're on the flat rate scheme, you still need to know the VAT element of your inclusive sales to apply the correct flat-rate percentage.
A dedicated tool makes this faster than doing the mental maths every time — for a reliable step-by-step walkthrough of the reverse method, see this guide to VAT backwards calculation.
The reduced and zero rates change the maths
Not everything is 20%. Domestic energy, children's car seats, and some energy-saving products sit at the 5% reduced rate — where you divide the gross by 21 to extract the VAT. Most food, children's clothes, and books are zero-rated, so the VAT element is simply nothing. The common trap is applying the standard "divide by 6" shortcut to a reduced-rate invoice and overstating the VAT — always check which rate applies first. HMRC publishes the full rate schedule, and it's worth bookmarking.
Rounding: the detail that trips people up
VAT calculations often produce fractions of a penny. HMRC allows you to round each line-item's VAT to the nearest penny, but you must be consistent — you can't round some lines up and others down to your advantage. If you invoice in accounting software, check how it rounds: per-line rounding and per-invoice rounding can differ by a penny or two, and those pennies add up across a quarter. When you're working VAT backwards manually, do the division first and round last.
Keep a record of how you got there
HMRC can ask how you arrived at your VAT figures up to four years later (six if they suspect carelessness). Save your workings: the gross amount, the rate applied, the formula or tool used, and the resulting net and VAT. A spreadsheet row per invoice is enough. If you've ever worked a figure backwards from a VAT-inclusive total, note that too — auditors are specifically interested in reverse calculations because that's where errors cluster.
The bottom line
Working VAT backwards is a two-second job once you know the rule: at 20%, divide the gross by 6. For anything else, use the formula gross × (rate ÷ (100 + rate)), and always confirm the rate before you start. Get this right and your VAT returns will reconcile cleanly — get it wrong and you'll either hand HMRC too much or too little. Neither is worth the risk when the maths is this simple.
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