Stacked vs additive discounts explained
Why an extra 10% off a 20% sale is not 30% off, how to read layered promotions, and when stacking genuinely beats a single big markdown.
The mistake almost everyone makes
The instinct is to add the percentages: a 20% coupon on a 20% sale must be 40% off. It is not. The second discount only ever bites into what is left after the first, so on a 100 item you pay 80, then 80% of that, landing at 64. That is a 36% effective discount, not 40. The bigger the individual cuts, the wider this gap grows, which is exactly why stores prefer to advertise the layers separately rather than the true combined figure.
How the math actually works
Each percentage discount is really a multiplier: 20% off means you keep 0.8 of the price, and 10% off means you keep 0.9. Chaining them multiplies the multipliers, so 0.8 times 0.9 is 0.72, meaning you pay 72% and save 28%. Because multiplication does not care about order, you can rearrange the discounts and the final price never changes, even though the per-step prices you see along the way will. This is the same logic the calculator runs for every row you add.
When stacking beats one big cut
Sometimes layered offers really are generous. A clearance rack at 50% off with an extra 20% coupon keeps 0.5 times 0.8, or 0.4 of the price, so you pay 40 on a 100 item, a genuine 60% off. Compare that effective figure against any rival's single flat discount rather than trusting the headline. The trap is only when a store dresses up a small second cut as if it doubled the deal, so always convert the stack to its one effective percentage before deciding.
Reading real-world promotions
Watch for the order of operations that a retailer chooses, because while the final price is the same, promotions sometimes exclude certain steps from certain items. A percentage coupon that applies before a fixed-dollar gift card behaves differently from one applied after, and loyalty cashback usually calculates on the post-discount total. When a deal mixes percentages with flat amounts or spend thresholds, split the percentage-only part into this calculator first, then apply the fixed pieces by hand to get the true out-the-door cost.