How to Calculate a Discount (Formula + Stacking Discounts)
A discount amount is the original price multiplied by the discount percentage, divided by 100. The sale price is what’s left after you subtract that amount, or equivalently, the original price multiplied by (100 minus the discount) divided by 100. Two numbers, one formula each, and most of the confusion at checkout comes from what happens when a second discount gets applied on top of the first.
The two formulas
Discount amount:
Discount = Original price × (Discount % / 100)Sale price:
Sale price = Original price × (100 − Discount %) / 100Both formulas need the same two inputs: the original price and the percentage off. Once you have the discount amount, the sale price is just the original price minus that number, so you rarely need to calculate both from scratch.
Worked example
Three items, three different discounts:
| Item | Original price | Discount | You save | Final price |
|---|---|---|---|---|
| Running shoes | $120.00 | 25% | $30.00 | $90.00 |
| Backpack | $65.00 | 40% | $26.00 | $39.00 |
| Headphones | $200.00 | 15% | $30.00 | $170.00 |
Notice the backpack and the headphones save the same $30, from very different discount percentages on very different prices. Percentage off tells you nothing about the dollar amount until you multiply it against the actual price, which is exactly why “40% off” on a cheap item can save you less than “15% off” on an expensive one.
Why stacked discounts don’t add up
Here’s where people get caught out: a store advertises 30% off, and you also have a 15% off coupon. It feels like 45% off. It is not.
Discounts stack multiplicatively, not additively, because the second discount applies to the already-reduced price, not the original one. Take a $250 item:
| Step | Price before | Discount applied | Amount off | Price after |
|---|---|---|---|---|
| Start | $250.00 | |||
| Apply 30% off | $250.00 | 30% | $75.00 | $175.00 |
| Apply 15% coupon | $175.00 | 15% | $26.25 | $148.75 |
The final price is $148.75, for a total discount of $101.25 off the original $250. As a percentage, that’s 40.5% off, not the 45% you’d get by simply adding 30 and 15. The gap, 4.5 percentage points, works out to $11.25 you didn’t actually save even though the two numbers on the tags and coupon said otherwise. The more discounts you stack, the wider that gap grows, since each one only ever touches what’s left, never the original number.
Flat discount vs. percentage discount: which wins?
Retailers also mix flat-dollar discounts (“$20 off”) with percentage discounts (“15% off”), and which one saves you more depends entirely on the price of the item.
A $20 discount saves exactly $20, always. A 15% discount saves more as the price rises. The two are equal at the price where 15% of the price equals $20, which is $20 / 0.15 = $133.33. Below that price, the flat discount wins; above it, the percentage wins.
| Item price | $20 off saves | 15% off saves | Better deal |
|---|---|---|---|
| $60.00 | $20.00 | $9.00 | Flat $20 |
| $100.00 | $20.00 | $15.00 | Flat $20 |
| $133.33 | $20.00 | $20.00 | Tied |
| $200.00 | $20.00 | $30.00 | 15% off |
| $400.00 | $20.00 | $60.00 | 15% off |
On cheaper items, chase the flat coupon. On big-ticket purchases, the percentage almost always wins, and the gap widens fast as the price climbs.
Calculate it with your own numbers
Enter the original price and pick a discount percentage below to see the exact amount you save and the final price. Change either value to compare offers side by side.
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Common mistakes
Adding stacked percentages together. As shown above, 30% off plus 15% off is not 45% off. Apply the first discount, then run the second discount on the new, lower price, never on the original.
Comparing percentage discounts across different prices without doing the math. “50% off” on a $40 item ($20 saved) can be a worse deal in absolute terms than “20% off” on a $150 item ($30 saved). The percentage alone doesn’t tell you what you’re actually keeping in your pocket.
Forgetting that “up to X% off” usually means only a few items hit that number. Sale signs advertise the highest discount in the group to catch your eye; most items in that same sale are discounted well below the headline percentage. Check the tag on the specific item, not the sign above the rack.
Assuming tax is calculated before the discount. In most places, sales tax applies to the discounted price, not the original one, so the final amount charged is usually a bit lower than “final price plus tax on the sticker price” would suggest. Confirm how your local tax rules handle this if you’re calculating an exact checkout total.
Frequently asked questions
How do I calculate a discount amount? Multiply the original price by the discount percentage and divide by 100. A $150 item at 40% off saves you $60, since 150 × 40 / 100 = 60.
How do I calculate the sale price directly, without finding the discount first? Multiply the original price by (100 minus the discount percentage), then divide by 100. That same $150 item at 40% off comes to 150 × 60 / 100 = $90.
Do two stacked discounts, like 30% and 15%, add up to 45% off? No. Apply the first discount, then apply the second to the resulting (lower) price rather than the original. A $250 item with 30% off then 15% off ends up at $148.75, a 40.5% total discount, not 45%.
Is a flat dollar discount or a percentage discount better? It depends on the item’s price. A flat discount saves the same dollar amount regardless of price, while a percentage discount scales with price. They break even at Flat amount / Percentage; below that price the flat discount wins, above it the percentage wins.