How to Calculate a Discount Percentage (Fast Mental Math)
Working out a sale price is one calculation almost everyone eventually does badly in their head — usually by rounding too aggressively, or by adding stacked discounts together instead of applying them one after another. Here's the fast, actually-correct way.
The basic formula
To find a discounted price: Sale price = Original price × (1 − discount as a decimal). A $50 item at 20% off is $50 × (1 − 0.20) = $50 × 0.80 = $40. To find just the amount saved rather than the final price: Savings = Original price × discount as a decimal — the same $50 item saves $50 × 0.20 = $10.
A shortcut for doing it in your head
Finding 10% of any number is just moving the decimal point one place to the left — 10% of $85 is $8.50. Once you have 10%, most common discount percentages are quick multiples or fractions of it: 20% is double the 10% figure, 5% is half of it, 30% is triple it, and 15% is the 10% figure plus half of it again. For $85 at 20% off: 10% is $8.50, so 20% is $17, and the sale price is $85 − $17 = $68.
Why stacked discounts don't add up the way you'd think
A common and costly mistake: assuming two discounts add together. They don't — each discount applies to whatever price is left after the previous one, not to the original price all over again. Two separate 20% discounts on a $100 item don't total 40% off (which would be $60); they leave you paying 100 × 0.80 × 0.80 = $64, a combined discount of 36%, not 40%. The gap grows larger the more discounts get stacked, and it always favors the seller relative to what a naive "just add the percentages" calculation would suggest.
Working backward: what discount did I actually get?
If you know the original price and the amount you saved (or the final price), you can work out the exact percentage: Discount % = (Amount saved ÷ Original price) × 100. Saved $15 on a $75 item? 15 ÷ 75 × 100 = 20% off. This is the calculation worth doing whenever a sale advertises "up to X% off" — the headline percentage often applies to only a subset of items, and working out the actual percentage on your specific item tells you what you really got.
Frequently asked questions
How do I calculate 20% off quickly in my head?
Find 10% by moving the decimal point one place left, then double it. For $85, 10% is $8.50, so 20% is $17, and the discounted price is $68.
Do two 20% discounts add up to 40% off?
No — stacked percentage discounts apply one after another, not added together. Two 20% discounts leave you paying 0.8 × 0.8 = 64% of the original price, a combined discount of 36%, not 40%.
Is 30% off then adding 30% back the same as the original price?
No. Taking 30% off a price and then adding 30% back never returns you to the original number, because the second 30% is calculated on a smaller starting amount.
How do I work out what percentage discount I actually got?
Divide the amount saved by the original price, then multiply by 100. Saving $15 on a $75 item is 15 / 75 × 100 = 20% off.