How to use it
- Sale price: enter the price and the percent off; add an extra discount for “an extra 10% off sale prices” offers and a sales tax rate if you want the total at the register.
- Percent off: enter the original and sale prices to see the discount as a percentage.
- Original price: enter the sale price and the percent off to work back to the price before the sale.
How it calculates
Money is held in whole cents and percentages to four decimal places, so every step is exact; each discount is rounded to the nearest cent with half a cent rounding up, as a register would. A second discount is taken from the already reduced price, which is why 25% then 10% comes to 32.5% off rather than 35%: the combined rate is 1 − (1 − 0.25) × (1 − 0.10). Sales tax, if entered, is charged on the final price, using the same rounding as our Sales Tax Calculator.
Working back to an original price, the calculator uses the fact that the sale price never goes down as the original price goes up, and finds by binary search every original price that rounds to the sale price you entered. At small discounts that is usually one price; at steep ones it is a run of neighbouring prices, and the calculator shows the lowest and highest.
What we tested
On October 14, 2026 we checked the page’s own functions against exact fractions on 4,004 random prices with one or two discounts and optional tax: every discount, sale price, saving, tax and total matched, every percent-off figure matched, and every original-price range matched an exhaustive search (2,698 of the cases had more than one possible original price). The first version searched only a few cents around the estimate and missed parts of those ranges at steep discounts; the test caught it before release. In Chromium: 25% off $80 showed $60.00 and $20.00 saved; adding an extra 10% and 7.25% tax showed $54.00 and $57.92, described as 32.5% off in total; $80 to $60 showed 25%; $41.99 at 30% off worked back to $59.98 to $59.99; $0.99 at 50% off showed the range $1.98 to $1.99; a 150% discount showed a message; the page fitted a 390 px phone with no errors.
| Example | Result |
|---|---|
| 25% off $80 | $60.00, you save $20.00 |
| 25% off $80, then 10% off, 7.25% tax | $54.00 (32.5% off in total), $57.92 with tax |
| $80 to $60 | 25% off |
| $59.99 at 30% off, original | $85.70 |
| $0.99 at 50% off, original | $1.98 to $1.99 |
Stores can apply stacked offers differently (both to the original price, or a coupon before a percentage), so check the terms of the offer.
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools