Discount & Sale Price Calculator
What you'll pay, what you'll save, and whether that second discount is as good as it sounds.
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The mental shortcuts that actually work in a shop
You rarely need the exact figure — you need to know quickly whether something is worth it. Build every discount out of 10%:
- 10% — move the decimal one place left. 10% of £45 is £4.50.
- 20% — double the 10%, or divide by five.
- 25% — divide by four. Often faster than the percentage route.
- 30% — 10% three times.
- 33% — divide by three, near enough.
- 50% — halve it.
- 15% — 10% plus half of that again.
Then subtract from the original. The one worth memorising is that the price you pay is always 100 minus the discount: at 30% off you pay 70%, so 0.7 × £45 is a single multiplication rather than two steps.
Why stacked discounts disappoint
"20% off, plus an extra 20% at the checkout" sounds like 40%. It is 36%.
The first discount takes £100 to £80. The second takes 20% off £80, not £100 — that's £16, not £20. You end up at £64, having saved 36%.
The gap widens as discounts grow. Three 30% discounts sound like 90% off but leave you paying 34.3% of the original — a saving of 65.7%. The rule: multiply the fractions you still pay (0.7 × 0.7 × 0.7), never add the percentages taken off.
Reading a sale honestly
"Was £120, now £60." Worth asking how long it was genuinely on sale at £120. Many jurisdictions require a reference price to have been charged for a minimum period, but enforcement is patchy.
"Up to 70% off." Legally satisfied if one item is. The average discount across a sale is usually far lower.
"Buy one get one free." A 50% discount, conditional on buying two. If you'd only have bought one, you haven't saved 50% — you've spent the same and got more.
"3 for 2." A 33% discount on three items, same conditionality.
Unit price is the only honest comparison
Packaging is designed to make comparison hard: 500g at £4.50 against 750g at £6.30. Divide each by its size and it's immediate — £0.0090 versus £0.0084 per gram, so the larger pack is about 6.7% cheaper per gram.
Two caveats. Supermarket shelf labels usually show unit price already, sometimes in inconsistent units (per 100g on one, per kg on the next). And a bulk saving on something perishable is a loss if you throw a third of it away.
Where the discount sits relative to tax
Discounts apply first, tax second. A £45 item at 30% off is £31.50, and 20% VAT on that is £6.30, giving £37.80. If tax were charged on the full £45, you'd pay £40.50 — nearly £3 more. Worth a glance at the receipt on a large purchase.
Frequently asked questions
How do I calculate a discount in my head?
Find 10% by moving the decimal one place left, then build from there. For 30% off £45: 10% is £4.50, so 30% is £13.50, leaving £31.50. For 25%, quarter it. For 15%, take 10% and add half of it again.
Do two 20% discounts make 40% off?
No — they make 36%. The second discount applies to the already-reduced price, so you pay 0.8 × 0.8 = 64% of the original. To combine discounts, multiply what remains rather than adding what is removed. The stacking mode below does this correctly.
Is the bigger pack always better value?
Usually, but not always, and shops rely on you assuming it. Use the unit-price mode to compare price per unit directly. Bear in mind that a saving on something you will not finish before it spoils is not a saving at all.
What does "up to 70% off" actually mean?
That at least one item somewhere in the shop is reduced by 70%. It says nothing about the rest. Advertising rules in most countries require only that the claim be true of some of the stock, so treat it as a headline rather than a description.
Should tax be applied before or after the discount?
After. The discount reduces the taxable amount, so tax is charged on the lower price. If a receipt shows tax computed on the pre-discount figure, you are being overcharged.