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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

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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.