The sale price formula
To find a sale price from an original price and a discount percentage: sale price = original price × (1 − discount ÷ 100). A $120 jacket at 30% off is $120 × 0.70 = $84. To work out how much you're saving in dollars rather than the final price, just subtract: $120 − $84 = $36 saved, or equivalently, original price × (discount ÷ 100).
Working backward: finding the discount percentage
If you know the original and sale price but want to know what percentage discount that represents, rearrange the formula: discount % = (original price − sale price) ÷ original price × 100. A $50 item on sale for $35 represents a ($50 − $35) ÷ $50 × 100 = 30% discount. This direction is especially useful for comparing two deals advertised differently — one as a percentage, one as a dollar amount — on the same basis.
Calculate a discount
Why stacked discounts don't simply add
"30% off, plus an extra 20% off" sounds like 50% total, but it isn't — the second discount applies to the already-reduced price, not the original. A $100 item at 30% off drops to $70; taking a further 20% off that $70 brings it to $56, a total discount of 44%, not 50%. The gap between the naive sum and the real combined discount grows as the individual percentages get larger, which is worth knowing before assuming two stacked promotions are as good as they sound added together.
Comparing a dollar-off deal to a percent-off deal
A flat "$20 off" and a "15% off" can't be compared directly without knowing the price, since a percentage's dollar value scales with the item's cost while a flat discount doesn't. On a cheaper item, the flat dollar amount is often the better deal; on a pricier item, the percentage usually wins. Running both through the actual formula — rather than eyeballing which sounds bigger — is the only reliable way to tell, especially since retailers tend to advertise whichever framing makes a given promotion look most appealing.