Discount calculator
Stacked discounts multiply rather than add. Twenty per cent off 100 leaves 80; a further ten per cent off that leaves 72, not 70. The effective discount is 28%, and the gap widens as the numbers get bigger — 50% then 50% is 75% off, not 100%. Retailers know this, which is why "an extra 20% off sale prices" reads better than the single number it actually amounts to.
A discount is the percentage multiplied by the price, then subtracted. Twenty per cent off 100 leaves 80. Two stacked discounts multiply rather than add: 20% then a further 10% leaves 72, an effective 28% off, not 30%.
How to calculate a discount
Working backwards from two prices is the more useful direction in a sale, because it exposes what a claimed discount is actually worth. A "50% off" that runs from an inflated reference price is a different offer from the same headline against a price the item genuinely sold at, and the only way to tell is to know what it cost before the sale started. In several jurisdictions the reference price must be the lowest price charged in the preceding thirty days for exactly this reason.
Questions
Multiply the price by the percentage and divide by 100, then subtract. Or multiply the price by (100 − discount) ÷ 100 in one step.