Discount Calculator

What is a discount calculator?

A discount calculator takes an original price and a percentage off, and returns the price you will actually pay along with how much you save. This one goes a step further: it also handles two discounts applied one after the other, like a storewide sale combined with a coupon at the till, and works out the single effective discount that combination amounts to.

Enter a price, choose a percentage using the quick buttons for 10, 20, 25, 30, 50 and 70 percent or type any other figure, and the sale price and your savings appear immediately. Turning on the second-discount option reveals a second percentage box for offers phrased as two separate cuts rather than one.

How to use the discount calculator

1. Enter the original price: type the full price before any discount into the price box.
2. Choose the discount: tap one of the quick buttons for 10, 20, 25, 30, 50 or 70 percent, or type any other figure into its own box. If the offer is two discounts applied one after another, turn on the second-discount option to reveal a second percentage box.
3. Read off the result: the final price, how much you save, and, when two discounts are stacked, the single effective discount they add up to appear at once, with a copy button to take the figures with you.

Why 20% off then 10% off is not 30% off

This is the fact this tool exists to make visible: stacked discounts do not add. “20% off, then a further 10% off at the till” sounds like 30% off, but the second discount applies to the already-reduced price, not the original one. Starting from 100, the first cut brings it to 80, and the second cut takes 10% off that 80, not off the original 100 — leaving 72. The effective discount is 28%, and you save 28 rather than the 30 the two numbers seem to promise. The gap always favours the shop.

The same pattern holds at other figures. 30% off followed by 15% off works out to an effective 40.5%, not the 45% you'd get by adding them. And two successive 50% discounts do not clear the price to zero: they leave 25 out of an original 100.

One thing does not change the outcome: the order. 30% then 15% and 15% then 30% land on exactly the same final price, because multiplying two percentages together gives the same result either way round. Checked across 361 pairs of discounts from 0 to 90 percent, the effective discount is never greater than the two percentages simply added together, and only equals that sum when one of the two discounts is zero. All the arithmetic here is done in exact fractions and rounded once, at the very end, so percentages of money never drift.

Use cases and honest limits

This tool is built for reading a sale sign correctly, working out what a coupon stacked on top of a storewide discount actually leaves you paying, and checking a two-stage offer before you get to the till. It's also useful for the moment a percentage-off deal and a fixed-amount-off deal both apply to the same item: the two swap places at a particular price, since 20% off beats 20 off anything above 100, and loses to it below 100.

It takes no view on tax. Whether a sale price is shown before or after sales tax or VAT depends on the country and the shop, so the calculator simply works on whatever number you type in, before or after tax as it applies to you. It also has no currency setting, because the arithmetic behind a percentage discount is identical whatever currency the price is in.

Why is it free?

The discount calculator runs entirely in your browser. Nothing you type is uploaded anywhere, there's no account to create, and there's no watermark on the result — just the price, the savings, and, if you stack two discounts, the effective rate behind them.