The Formula
A percentage off is the simplest arithmetic in retail, and the one most often done wrong at the shelf:
Sale price = Original × (1 − Discount / 100)
The fastest mental method for common discounts: 10% is the price with the last digit dropped; 20% is double that; 25% is a quarter, so divide by four; 30% is 10% tripled. For 15%, take 10% and add half of it.
Discount Chart
| Discount | On $50 | On $100 | On $250 | On $500 |
|---|---|---|---|---|
| 10% | $45.00 | $90.00 | $225.00 | $450.00 |
| 20% | $40.00 | $80.00 | $200.00 | $400.00 |
| 25% | $37.50 | $75.00 | $187.50 | $375.00 |
| 30% | $35.00 | $70.00 | $175.00 | $350.00 |
| 40% | $30.00 | $60.00 | $150.00 | $300.00 |
| 50% | $25.00 | $50.00 | $125.00 | $250.00 |
| 70% | $15.00 | $30.00 | $75.00 | $150.00 |
Stacked Discounts Do Not Add
"30% off, then an extra 20% at checkout" is not 50% off. The second discount applies to the already-reduced price:
$100 → 30% off → $70 → 20% off → $56 (44% off, not 50%)
The general rule: combined discount = 1 − (1 − a)(1 − b). Two 50% discounts give 75%, never 100%. The order does not matter; the result is the same either way.
A Fixed Coupon Behaves Differently
A $20-off coupon is worth more on a cheap item than an expensive one in percentage terms, and applying it before or after a percentage discount changes the total. Retailers almost always apply percentage discounts first, which minimises the coupon's effect. Where you can choose, applying the fixed amount first is better for you.
Reading a Sale Honestly
"Was $200, now $120" only means something if $200 was a real price. U.S. FTC guidance and various state laws require a former price to have been genuinely offered for a reasonable period, but enforcement is limited and permanent-sale pricing is widespread. The useful question is what the item is worth to you, not what the tag claims it was.
Frequently Asked Questions
How do I calculate the discount percentage from two prices?
Subtract the sale price from the original, divide by the original, multiply by 100. From $129.99 to $90.99 is $39.00 / $129.99 = 30%.
Is sales tax charged before or after the discount?
After. Tax is calculated on what you actually pay, so a discount reduces the tax as well. Manufacturer coupons are the exception in some states, where tax is charged on the pre-coupon price.
What does "up to 70% off" mean?
That at least one item is 70% off. It is the ceiling of the range, not the typical discount, and it usually applies to the least desirable stock.