Stacked Discount Calculator

Why stacked discounts don't add up

Applying 20% off followed by 10% off is not the same as taking 30% off in one step. Each discount is calculated on whatever the price has already been reduced to, not on the original price, so a $100 item at 20% off becomes $80, and 10% off that $80 is $8, landing at $72, an effective discount of 28%, not 30%. The gap between the naive sum and the real effective discount grows as you stack more percentages or use larger ones.

Why the order can matter

For pure percentage discounts, the final price comes out the same no matter what order you apply them in, since multiplication is commutative: 20% then 10% gives the identical result as 10% then 20%. The one exception here is the flat dollar discount, which this calculator always applies last, after the percentage discounts. Applying a flat discount first instead of last changes the result, since a percentage taken after a flat discount is calculated on a smaller base.

Also known as a double discount calculator

When only two percentage discounts are involved, this same math is often called a double discount calculator. The formula and the underlying reason it works the way it does are identical whether you are stacking two discounts or three: each one multiplies the running price by (1 minus that discount as a decimal).