Black Friday: “70% OFF EVERYTHING!” You grab three items, head to checkout, and the total is somehow still $180. How is that possible? Understanding discount math protects your wallet from misleading sales tactics.
Basic Discount Calculation
The simplest discount calculation:
Sale Price = Original Price × (1 - Discount Rate)
Or if you prefer: Sale Price = Original Price - (Original Price × Discount %)
Example: A $95 jacket at 40% off:
- $95 × (1 - 0.40) = $95 × 0.60 = $57
- You save: $95 - $57 = $38
Quick shortcut: To find the price after discount, just multiply by the “keep” percentage:
- 20% off → multiply by 0.80
- 25% off → multiply by 0.75
- 30% off → multiply by 0.70
- 40% off → multiply by 0.60
- 50% off → multiply by 0.50
Stacked Discounts (The Tricky Part)
“Take an extra 20% off already reduced items!” Sounds amazing, but the math isn’t what most people think.
The wrong way: 40% off + 20% off = 60% off ❌
The right way: Apply each discount sequentially.
Example: $100 item, 40% off then extra 20% off:
- First discount: $100 × 0.60 = $60
- Second discount: $60 × 0.80 = $48
- Actual total discount: ($100 - $48) ÷ $100 = 52% off
That’s 52% off, not 60%. The difference adds up when you’re spending hundreds of dollars.
Quick formula for stacked discounts: Total discount = 1 - (first multiplier × second multiplier) = 1 - (0.60 × 0.80) = 1 - 0.48 = 52%
Finding the Original Discount Percentage
Sometimes you see a sale tag but want to know the actual percentage off:
Discount % = ((Original - Sale Price) ÷ Original) × 100
Example: Was $249, now $179:
- ($249 - $179) ÷ $249 × 100 = 28.1% off
This is useful when stores just show the sale price without stating the percentage.
Sales Tax After Discount
Remember: sales tax is calculated on the discounted price, not the original price. That’s one small benefit of buying on sale.
Example: $80 item at 25% off in a state with 8% sales tax:
- Discounted price: $80 × 0.75 = $60
- Tax: $60 × 0.08 = $4.80
- Total: $64.80
”Buy One Get One” Deals Decoded
BOGO deals aren’t always 50% off:
- BOGO Free — Buy 1, get 1 free = 50% off if you buy exactly 2
- BOGO 50% off — Buy 1, get 1 at half price = 25% off per item
- Buy 2 Get 1 Free — 33% off per item (if you buy exactly 3)
The real question is whether you actually need the extra items. A “deal” on something you don’t need isn’t a deal — it’s spending.
Calculate Any Discount Instantly
Skip the mental math. Our free discount calculator shows you the sale price, savings amount, and final price with tax in seconds. For more complex math, check out our percentage calculator.