Multiply by what is left, not by what comes off. A 30% discount leaves 70% of the price, so a 120 item costs 120 × 0.70 = 84. That is one multiplication with nothing to carry and nothing to subtract from the wrong number. In your head it is easier still: 10% of 120 is 12, three of those is 36, and 120 − 36 = 84.
Why multiply instead of subtract?
Both routes reach the same answer. The difference is how many chances you get to go wrong.
Subtracting means two operations: work out the discount, then take it away. Every step is a place to lose a decimal. Multiplying by the survivor — 0.70 for 30% off, 0.85 for 15% off, 0.65 for 35% off — is one operation, and the number you type is the number you pay.
It also keeps the question straight. "70% of the price remains" is harder to misread than "30% off", and thinking in survivors is what makes the stacked cases below behave.
How to do it in your head
Everything is built from 10%, which is the decimal point moved one place left. 10% of 48 is 4.80. 10% of 260 is 26. From there:
- 20% — double the 10%.
- 5% — half the 10%.
- 15% — the 10% plus the 5%.
- 25% — halve the price, then halve again.
- 1% — the decimal point moved two places, for building the awkward ones.
So 35% off 60: 10% is 6, three of those is 18, half of 6 is 3, total 21. You pay 39.
For the percentages that are not built from fives, round and then correct. 18% off 45: take 20%, which is 9, then give back the 2% you overshot, which is 0.90. The discount is 8.10 and you pay 36.90. Overshooting and repairing with a 1% multiple is nearly always faster than attacking 18% head on.
Is 30% off plus an extra 20% off the same as 50% off?
No, and this is the one that costs people money at the till. The second discount comes off the already-reduced price, so it is smaller than the sign suggests. Multiply the survivors: 0.70 × 0.80 = 0.56. You keep 56% of the price, which is 44% off, not 50%. On a 120 item that is 67.20 rather than 60.
The gap widens as the numbers grow. Two stacked 50% discounts give 75% off, not 100%. You can keep piling percentage discounts on forever and the item never becomes free, because each one only ever takes a slice of what survived the last. Only a single, genuine 100% gets you to zero.
If you want the figure without the mental arithmetic, the discount calculator here has a second discount field and prints the effective percentage next to the price, so you can see what "extra 20% off sale prices" is really worth. It stacks two discounts and no more: cashback, loyalty points and free-shipping thresholds are not percentages of the item price and do not belong in the same multiplication.
Does the order of two discounts matter?
For two percentages, no. Multiplication does not care which order you do it in, so 30% then 20% and 20% then 30% both land on 0.56.
The moment a fixed amount joins the party, order matters a lot. Take a 100 item with a 10-off voucher and a 20% sale:
- Voucher first: 100 − 10 = 90, then 20% off = 72.
- Percentage first: 100 × 0.80 = 80, then − 10 = 70.
Two pounds, dollars or euros between them, and you do not get to choose: the checkout applies them in whatever order it was set up for. The gap grows with the size of the voucher, so on anything more than a token amount it is worth reading the receipt rather than assuming.
How do I find the original price from a sale price?
Divide by what is left. If you paid 84 after 30% off, the price was 84 ÷ 0.70 = 120.
The tempting move — adding 30% back to 84 — gives 109.20, and it is wrong every time. The discount was taken from the larger number, so adding the same percentage to the smaller one always comes up short. The error is not tiny either: 10.80 on a 120 item, about 9%.
The related question, "what percent off is this really", works the same way round. Subtract to get the saving, then divide by the original price: 120 − 84 = 36, and 36 ÷ 120 = 0.30, so 30% off. Divide by the sale price instead and you get 42.9%, which is a flattering number with no meaning. Choosing the wrong base is the single most common percentage mistake there is, and working out percentages without losing track of the base covers the rest of the family: increases, decreases, and why a 20% fall does not cancel a 20% rise.
What is a multibuy worth as a percentage?
Offers that are not written as percentages usually are one underneath, provided you wanted all the items:
- Three for two — one free in three, so 33.3% off.
- Buy one get one free — 50% off across the pair.
- Buy one get one half price — 25% off across the pair.
- Buy two get one half price — 16.7% off across the three.
That qualifier is the whole offer. A three-for-two on something you would have bought once is not 33% off; it is a 100% increase in what you spend, arranged so that it feels like a saving.
Does tax change the discount?
It depends on where the price is quoted. In VAT countries the shelf price already includes the tax, so a percentage off the shelf price is the percentage you get and the figure you calculate is the figure you hand over. In the US, sales tax is added at the till after the discount, so the real total is higher than any discount calculation will tell you. If you need the invoice numbers rather than the sticker ones, adding or stripping the tax is a separate step.
Is the "was" price real?
The percentage is only worth as much as the price it is measured against. In the EU, the Omnibus Directive requires an announced price reduction to state the prior price, defined as the lowest price the seller charged in the previous 30 days, precisely because a reference price nobody ever paid makes the discount fictional. If something has been "40% off" since spring, the sale price is simply the price, and the percentage is decoration.
Why your figure and the receipt disagree
Tills round in their own way. Some round the discount down, some round the final price, some round cash totals to the nearest 0.05 where small coins have been withdrawn from circulation. Expect a cent of disagreement now and then. If the gap is bigger than that, it is usually the order-of-operations problem above, not your arithmetic.
The mental method covers most of what you need standing in a shop, and the discount calculator covers the rest: it stacks two discounts, works backwards from a sale price to the original, and shows the arithmetic underneath so you can check it rather than trust it.
If the base kept moving under you while you read this, that is the real problem and it is worth fixing properly. How to calculate percentages without getting lost deals with increases, decreases and percentage points, which is the machinery every discount sits on top of.
Frequently asked questions
How do I calculate a 20% discount?
Multiply the price by 0.80, because 20% off leaves 80% of it. A 45 item costs 45 × 0.80 = 36. In your head: 10% of 45 is 4.50, double it to 9, and take 9 off 45.
How do I work out a discount in my head?
Find 10% by moving the decimal point one place left, then build the percentage you need out of it. 20% is double the 10%, 5% is half of it, and 15% is the two added together. For 35% off 60: 18 plus 3 is 21 off, so you pay 39.
Is 50% off plus an extra 20% off the same as 70% off?
No. The extra 20% comes off the already-halved price, so you keep 0.50 × 0.80 = 0.40 of the original. That is 60% off, ten points short of what the two signs add up to. Stacked percentage discounts can never reach 100%.
What was the original price before the discount?
Divide the price you paid by what was left after the discount. Paying 84 after 30% off means 84 ÷ 0.70 = 120. Adding 30% back onto 84 gives 109.20, which is wrong, because the discount came off the larger number.
Is sales tax added before or after a discount?
In the US, the discount is applied first and sales tax is calculated on the reduced price at the till, so your total is higher than the discounted figure. In VAT countries the shelf price already includes the tax, so the discounted price is what you hand over.
Last updated September 19, 2026