How to Calculate a Discount, and Why 20% Plus 10% Is Not 30%

A sign says "25% off" and you want to know what you'll actually pay. The maths is simple once you see it, and there's one trap — stacked discounts — that catches almost everyone.

Red sale tags on a red background

The basic formula

To find the price after a discount:

Final price = original price × (1 − discount ÷ 100)

For a $80 jacket at 25% off: 80 × (1 − 0.25) = 80 × 0.75 = $60. You save $20.

A quick mental shortcut: 10% is the original price with the decimal point moved one place left. Take 10% of $80 ($8), double it for 20% ($16), and add half of 10% for 25% ($20).

Working backwards: what percent off is it?

Sometimes the shop shows the old and new price but not the percentage. Subtract the sale price from the original, divide by the original, and multiply by 100:

Percent off = (original − sale) ÷ original × 100

Was $120, now $90: (120 − 90) ÷ 120 = 0.25, so it's 25% off.

Why 20% plus 10% is not 30%

Offers like "20% off, plus an extra 10% off at the till" apply the second discount to the already reduced price:

$100 with 20% off = $80
$80 with a further 10% off = $72

You paid $72, so the total discount is 28%, not 30%. Each new discount is worth a little less than the one before because it applies to a smaller number.

How to calculate it in seconds

Open the Discount Calculator, enter the original price and the discount, and add a second discount if there is one. It shows the final price, how much you save and the combined percentage. Switch to "The discount % from two prices" to work out how good a deal really is.

A calculator and shopping receipts on a table

Common mistakes

Adding stacked discounts together. 20% + 10% is 28%, not 30%.
Taking the discount off the wrong number. A percentage of the sale price is smaller than a percentage of the original.
Forgetting tax. Prices in some countries are shown before tax; the discount usually applies before tax is added.
Confusing "up to 50% off" with "50% off". "Up to" means only some items are reduced that much.

FAQ

How do I calculate 30% off? Multiply the price by 0.70. A $50 item at 30% off costs $35.

How do I find the original price from the sale price? Divide the sale price by (1 − discount). If something costs $60 after 25% off, the original was 60 ÷ 0.75 = $80.

Is a discount applied before or after tax? Usually before — the tax is then calculated on the reduced price. Check the shop's terms if it matters.

Try it now

Use the free Discount Calculator — no sign-up. For other percentage questions, try the Percentage Calculator.