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How to Calculate a Discount
The formulas for percent off, sale price, and discount percentage — with worked examples and mental math shortcuts.
The three core formulas
Sale price from percent off
Sale price = Original price × (1 − Discount% / 100) Example: $80 item, 25% off Sale price = $80 × (1 − 0.25) = $80 × 0.75 = $60
Savings amount
Savings = Original price × (Discount% / 100) Example: $80 item, 25% off Savings = $80 × 0.25 = $20
Discount percentage from prices
Discount% = (Original − Sale) / Original × 100 Example: was $80, now $60 Discount% = ($80 − $60) / $80 × 100 = 25%
Mental math shortcuts
| Discount | Mental math trick |
|---|---|
| 10% off | Move decimal one place left ($80 → $8 off) |
| 20% off | Find 10%, double it ($80 → $8 × 2 = $16 off) |
| 25% off | Divide by 4 ($80 ÷ 4 = $20 off) |
| 50% off | Divide by 2 ($80 ÷ 2 = $40 off) |
| 75% off | Find 25%, subtract from original ($80 − $20 = $60 off) |
| 15% off | Find 10%, add half ($8 + $4 = $12 off) |
Stacked discounts
When two discounts are applied in sequence, they don't simply add. A 20% off followed by another 10% off is not 30% off:
$100 item, 20% off then 10% off: Step 1: $100 × 0.80 = $80 (after 20% off) Step 2: $80 × 0.90 = $72 (after 10% off) Total discount: $28 off — not $30 off Combined rate: 1 − (0.80 × 0.90) = 1 − 0.72 = 28%
Discount Calculator
Calculate sale prices, savings, and discount percentages instantly.
Frequently asked questions
- How do I calculate percent off in my head?
- For most discounts, find 10% first (move the decimal left), then scale. For 30% off $45: 10% = $4.50, so 30% = $4.50 × 3 = $13.50 off. Sale price = $45 − $13.50 = $31.50.
- Is "buy one get one 50% off" better than "25% off both"?
- They're identical in total savings. BOGO 50% on two $20 items: you pay $20 + $10 = $30 (saving $10). 25% off both: $15 + $15 = $30 (saving $10). Retailers use different framing to make the deal sound more appealing.
- How do I calculate the original price from a sale price?
- Divide the sale price by (1 − discount%/100). If an item is $60 after a 25% discount: original = $60 / 0.75 = $80.