How to Calculate a Discount
A discount is a percentage reduction from an original price. Once you know the two-step formula, you can calculate any sale price in your head -- no calculator needed.
The discount formula
To find the sale price, calculate the discount amount first, then subtract it from the original price. There are two ways to express this:
These two steps combine into a single formula if you prefer to go straight to the sale price:
Example: A $120 jacket is 30% off. Discount amount = $120 x 0.30 = $36. Sale price = $120 - $36 = $84. Using the combined formula: $120 x (1 - 0.30) = $120 x 0.70 = $84.
Key insight
Multiplying by (1 - discount decimal) gives you the sale price in one step. A 25% discount means you pay 75% of the original price. A 40% discount means you pay 60%. The "pay percentage" always adds up to 100% with the discount percentage.
Mental math shortcuts
You can estimate most common discounts in your head using a few reliable tricks.
10% off: Move the decimal one place left. On a $75 item, 10% off = $7.50. Sale price = $67.50.
20% off: Find 10% and double it. On a $75 item, 10% = $7.50, doubled = $15. Sale price = $60.
25% off: Divide the price by 4. On a $80 item, $80 / 4 = $20 off. Sale price = $60.
50% off: Divide the price by 2. On a $90 item, $90 / 2 = $45. Sale price = $45.
30% off: Find 10% and multiply by 3. On a $70 item, 10% = $7, times 3 = $21 off. Sale price = $49.
15% off: Find 10%, then add half of that. On a $60 item, 10% = $6, half = $3, total off = $9. Sale price = $51.
For any other percentage
Break it into parts you can calculate quickly. For 35% off, calculate 30% off and add 5% (half of 10%). On a $100 item: $30 + $5 = $35 off. Sale price = $65. The same decomposition works for any awkward percentage.
Discount quick-reference table
Common original prices with 10%, 20%, 25%, and 50% discounts already calculated.
| Original | 10% off | 20% off | 25% off | 50% off |
|---|---|---|---|---|
| $20 | $18.00 | $16.00 | $15.00 | $10.00 |
| $30 | $27.00 | $24.00 | $22.50 | $15.00 |
| $40 | $36.00 | $32.00 | $30.00 | $20.00 |
| $50 | $45.00 | $40.00 | $37.50 | $25.00 |
| $60 | $54.00 | $48.00 | $45.00 | $30.00 |
| $75 | $67.50 | $60.00 | $56.25 | $37.50 |
| $80 | $72.00 | $64.00 | $60.00 | $40.00 |
| $100 | $90.00 | $80.00 | $75.00 | $50.00 |
| $120 | $108.00 | $96.00 | $90.00 | $60.00 |
| $150 | $135.00 | $120.00 | $112.50 | $75.00 |
| $200 | $180.00 | $160.00 | $150.00 | $100.00 |
| $250 | $225.00 | $200.00 | $187.50 | $125.00 |
| $300 | $270.00 | $240.00 | $225.00 | $150.00 |
| $500 | $450.00 | $400.00 | $375.00 | $250.00 |
For prices or discount percentages not listed here, use the Discount Calculator for an instant result with the exact dollar saving.
Checking a discount at the register
Sale signage and register behaviour do not always agree. An item can be tagged at one discount, ring up at another, or apply the reduction to a base price you were not expecting. Working the calculation backwards from your receipt tells you what discount you actually received.
Example: the tag said 30% off and the original price was $90. You paid $63. Divide 63 by 90 to get 0.70, subtract that from 1 to get 0.30, and multiply by 100. The discount was 30%, exactly as advertised.
If the number comes back lower than the sign promised, the usual explanations are that the discount excluded certain brands or categories, that it applied to a base price other than the one on the tag, or that a promotion had already expired. All three are worth raising at the counter before you leave, since the register receipt is the evidence.
The same arithmetic works as a comparison tool. Two stores advertising the same headline percentage off different starting prices are not offering the same deal, and the price you actually pay is the only figure that settles it.
Stacked discounts and coupon codes
Many retailers offer a base sale discount plus an additional coupon code. These do not add together -- they compound sequentially, which means the real saving is always less than the sum of the two percentages.
Example: A $100 item is 30% off, then you apply an extra 20% off coupon.
- 1Apply first discount: $100 x 0.70 = $70
- 2Apply second discount to the new price: $70 x 0.80 = $56
- 3Total saving: $100 - $56 = $44 (44% off, not 50%)
The stacking formula
Combined effective discount = 1 - (1 - d1) x (1 - d2). For 30% and 20%: 1 - (0.70 x 0.80) = 1 - 0.56 = 0.44 = 44%. The gap between the headline number (50%) and the real saving (44%) is how retailers make stacked deals sound more attractive than they are.
How to evaluate whether a deal is actually good
A high discount percentage does not always mean a large saving in dollar terms. Here is a reliable three-step framework:
- 1Calculate the actual dollar saving: A 50% discount on a $20 item saves $10. A 20% discount on an $80 item saves $16. The dollar amount is what matters for your budget, not the percentage headline.
- 2Compare the sale price to alternatives: Check the same or equivalent product at other retailers before assuming a discount is competitive. A 30% off sticker on an already-inflated "original" price can result in paying more than the everyday price elsewhere.
- 3Factor in tax before deciding: If your state has sales tax, the final price is the sale price plus tax -- not just the discounted sticker. An $80 sale price with 9% tax becomes $87.20. For large purchases, this gap matters.
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