Black Friday math is designed to confuse: "30% off, then an extra 20% at checkout!" sounds like 50% off, but it is not — discounts multiply. This calculator shows the real sale price, your real savings, and what stacked offers actually deliver.
The basic calculation
Sale price = Original × (1 − discount%). A $200 item at 30% off costs $140 and saves $60. Percent-off means you pay (100 − discount)% of the original: 30% off = paying 70%.
Stacked discounts multiply
30% off followed by an extra 20% off is 1 × 0.7 × 0.8 = 0.56 — you pay 56%, so the combined discount is 44%, not 50%. The order does not matter (multiplication commutes), which surprises many shoppers. Coupon apps sometimes apply "extra 20%" before the 30% — same result, but check.
Buy-one-get-one math
BOGO 50% off is effectively 25% off per item when you buy two identical items. BOGO free on identical items is 50% off per item. Retailers love these framings because they sound bigger than the equivalent percentage.