Stacked discounts do not add up
An advertised "50% off, plus an extra 20% off" is not 70% off. The second discount applies to the already-reduced price, so an item at 100 goes to 50, then to 40 — a 60% total reduction, not 70%.
The general rule: two discounts of a% and b% combine to a total of a + b − (a×b÷100) percent. The effective figure above does this for you, which is usually the number worth comparing across stores.
Working out the original price
To recover the pre-discount price from a sale price, divide by one minus the discount as a decimal. An item marked 30% off and selling for 70 was originally 70 ÷ 0.7 = 100.
This is worth doing when a retailer advertises a percentage off an unstated "original" price. Comparing the actual sale price against other stores is more informative than the size of the discount claimed.
Discount tactics worth recognising
Anchoring is the most common: a high reference price makes the sale price feel like a bargain regardless of whether anything ever sold at the higher figure. Several jurisdictions now require that the reference price was genuinely charged for a minimum period.
"Buy one get one free" is a 50% discount if you wanted two items and a 0% discount if you wanted one. "Buy two get one free" is 33.3% off, not 50%. Converting every offer into a per-unit price is the fastest way to compare them honestly.
Frequently asked questions
Does the order of two discounts matter? +
No. Multiplication is commutative, so 20% then 50% gives exactly the same final price as 50% then 20%.
How do I calculate 30% off? +
Multiply the price by 0.70, or work out 30% and subtract it. Both give the same answer; multiplying by the remainder is fewer steps.
Is 50% off then 20% off the same as 70% off? +
No — it is 60% off. The second discount applies to the already-reduced price, not the original.