Double Discount Calculator

Here is the trick that trips up almost everyone at the register: stacking a 20% discount on top of a 15% discount does not give you 35% off. It gives you 32%. Discounts multiply, they do not add.

The reason is that the second discount applies to the already-reduced price, not the original. Take a $100 item. The 20% off brings it to $80. Then 15% off that $80 is $12, not $15, so you land at $68. That is a 32% total saving, not 35%. The more discounts you stack, the wider the gap grows between what the percentages add up to and what you actually save.

The formula is simple once you see it. Multiply the original price by (1 minus each discount), one after another. For two discounts of d1 and d2, the final price is Price times (1 minus d1) times (1 minus d2). To get the single equivalent discount, work out 1 minus (1 minus d1) times (1 minus d2). For the 20% and 15% example that is 1 minus 0.8 times 0.85 = 0.32, or 32%.

This matters more than it sounds. Store coupons that say “extra 30% off sale prices,” cashback on top of a promo code, wholesale tiers, and “stack these codes” deals all work this way. Order does not change the final price, so do not fret about which discount goes first. What changes is your expectation: the headline numbers always look better added up than they really are. This calculator shows the actual final price, the true effective discount, and how much each step takes off, so you know what you are really paying before you reach the till.