How to Calculate Percent Off
The formula for calculating any percentage discount — plus worked examples, mental math shortcuts, and a quick reference table for common prices and discounts.
The Step-by-Step Method
Calculating a percentage discount is a two-step process. You first find the savings amount, then subtract it from the original price to get the sale price. Or you can do it in a single multiplication by converting the discount into a "keep" factor — which is usually faster in practice.
Worked Example: 30% Off $80
The "Keep Factor" Explained
The most important insight for calculating percent off is that X% off means you pay (100 − X)% of the original price. This means you can always multiply by a single number:
| Discount | Keep Factor | Example: $100 item |
|---|---|---|
| 10% off | × 0.90 | $90.00 |
| 15% off | × 0.85 | $85.00 |
| 20% off | × 0.80 | $80.00 |
| 25% off | × 0.75 | $75.00 |
| 30% off | × 0.70 | $70.00 |
| 40% off | × 0.60 | $60.00 |
| 50% off | × 0.50 | $50.00 |
| 75% off | × 0.25 | $25.00 |
Once you internalize the keep factor, most percent-off calculations become mental arithmetic. For instance, if a jacket is 40% off and costs $120, you know you're paying 60% of $120 — which is $72 — without needing a calculator.
Quick Reference Table
Sale prices for common original price and discount combinations. All amounts rounded to the nearest cent.
| Original Price | 10% off | 20% off | 25% off | 30% off | 50% off |
|---|---|---|---|---|---|
| $20 | $18.00 | $16.00 | $15.00 | $14.00 | $10.00 |
| $25 | $22.50 | $20.00 | $18.75 | $17.50 | $12.50 |
| $30 | $27.00 | $24.00 | $22.50 | $21.00 | $15.00 |
| $40 | $36.00 | $32.00 | $30.00 | $28.00 | $20.00 |
| $50 | $45.00 | $40.00 | $37.50 | $35.00 | $25.00 |
| $60 | $54.00 | $48.00 | $45.00 | $42.00 | $30.00 |
| $75 | $67.50 | $60.00 | $56.25 | $52.50 | $37.50 |
| $80 | $72.00 | $64.00 | $60.00 | $56.00 | $40.00 |
| $100 | $90.00 | $80.00 | $75.00 | $70.00 | $50.00 |
| $120 | $108.00 | $96.00 | $90.00 | $84.00 | $60.00 |
| $150 | $135.00 | $120.00 | $112.50 | $105.00 | $75.00 |
| $200 | $180.00 | $160.00 | $150.00 | $140.00 | $100.00 |
| $250 | $225.00 | $200.00 | $187.50 | $175.00 | $125.00 |
| $300 | $270.00 | $240.00 | $225.00 | $210.00 | $150.00 |
| $500 | $450.00 | $400.00 | $375.00 | $350.00 | $250.00 |
Mental Math Shortcuts
You won't always have a phone handy in a store. These shortcuts let you estimate any common discount in seconds — accurate enough for a buying decision.
Finding What Percentage Off You're Getting
Sometimes you know both the original price and the sale price and want to find the discount percentage — for example, when a price tag shows a "was / now" comparison. The formula is: ((Original − Sale) ÷ Original) × 100. This is the standard percentage change formula applied to a price decrease.
Example: A shirt was $45, now $31.50. ((45 − 31.50) ÷ 45) × 100 = (13.50 ÷ 45) × 100 = 30%. It's 30% off. The percent off calculator handles this automatically if you enter both prices.
Real-World Example: Buying a Coat
A winter coat is originally priced at $180. The store is running a "25% off everything" sale, and you have an extra coupon for an additional 10% off the sale price. How much do you pay?
Step 1 — 25% off $180: $180 × 0.75 = $135. Step 2 — 10% off the new price: $135 × 0.90 = $121.50. You pay $121.50, a total saving of $58.50. Note that 25% + 10% = 35%, but you're not getting 35% off the original — you're getting only 32.5% off ($58.50 ÷ $180 × 100 = 32.5%). The stacked discount calculator shows this breakdown for up to 5 sequential discounts.
Why Sequential Discounts Don't Add Up
The most common misconception about percent-off calculations is that stacking discounts adds their percentages. If a store offers 20% off, and you have a 10% coupon, many shoppers expect 30% off. The actual result is 28% off. After the first 20% discount you're paying 80% of the original. The second 10% off comes off that reduced price: 80% × 90% = 72% of the original — meaning a 28% total discount. The gap between "30%" and "28%" seems small on a $50 item ($1 difference) but is noticeable on a $500 purchase ($10 difference).
Applying Discounts Before or After Tax
In the US, discounts are typically applied to the pre-tax price. Sales tax is then calculated on the discounted amount. So if a $100 item is 20% off, you pay $80 + tax on $80 — not $100 + tax then a $20 rebate. For purchases where both a discount and sales tax apply together, the discount plus tax calculator handles both in one step. In some countries, particularly those with VAT, the discount may be applied to the VAT-inclusive price — always check the displayed price includes or excludes tax.
Finding the Original Price from a Sale Price
If you know the sale price and the discount percentage and want to work backwards to the original price, do not add the discount percentage back to the sale price — that gives the wrong answer. Instead, divide by the keep factor: Original = Sale Price ÷ (1 − Discount% ÷ 100). For example, a $63 sale price with a 30% discount: Original = $63 ÷ 0.70 = $90. Adding 30% to $63 would give $81.90 — $8.10 short of the right answer. A full explanation of this reverse calculation is on the original price before discount guide.
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