Sales, markdowns, promotional offers, and bundle deals bombard us constantly -- and retailers know that most shoppers make discount decisions based on intuition rather than math. A few simple percentage skills can help you make better purchasing decisions in seconds, whether you're at a department store, shopping online, or comparing competing products.
The Basic Discount Formula
To find the sale price after a percentage discount: Sale price = Original price x (1 minus discount%). Examples:
- $80 jacket, 25% off: $80 x 0.75 = $60
- $150 shoes, 40% off: $150 x 0.60 = $90
- $35 book, 15% off: $35 x 0.85 = $29.75
Mental Math Shortcuts for Common Discounts
- 10% off: Move decimal left one place. $90 becomes $81 (subtract $9)
- 20% off: Find 10%, double it, subtract. $90 -- subtract $18 -- $72
- 25% off: Divide by 4, subtract. $80 -- $20 off -- $60
- 50% off: Halve the price. $120 -- $60
- 30% off: Find 10%, multiply by 3, subtract. $70 -- $21 off -- $49
- 15% off: Find 10%, add half of that. $60 -- $6 + $3 = $9 off -- $51
Comparing Two Different Discounts
Store A has a $120 item at 30% off. Store B has a similar item at $90 with $40 off. Which is cheaper?
- Store A: $120 x 0.70 = $84.00
- Store B: $90 - $40 = $50.00
Store B is dramatically cheaper. The lesson: "30% off" sounds impressive on a premium-priced item, while "$40 off" on a cheaper item is actually the much better value. Always calculate the final price.
Stacked Discounts: They Don't Add Up the Way You Think
If a store advertises "an extra 20% off already-discounted prices," many people assume that's 40% total. It's not.
- Original price: $100
- First discount (20% off): $80
- Second discount (20% off $80): $64
- Total effective discount: 36%, not 40%
The formula for stacked discounts: Effective discount = 1 minus (1 minus d1) x (1 minus d2). Two 20% discounts = 1 - (0.80 x 0.80) = 36% effective discount.
Sales Tax: The Discount Offset
A 10% discount followed by 8% sales tax doesn't get you back to the original price. Original: $100. After 10% off: $90. After 8% tax: $90 x 1.08 = $97.20. Factor this in when comparing "no tax" promotions against a percentage discount.
A Quick Comparison Table
| Original Price | 10% Off | 20% Off | 30% Off | 50% Off |
|---|---|---|---|---|
| $25 | $22.50 | $20.00 | $17.50 | $12.50 |
| $50 | $45.00 | $40.00 | $35.00 | $25.00 |
| $100 | $90.00 | $80.00 | $70.00 | $50.00 |
| $200 | $180.00 | $160.00 | $140.00 | $100.00 |
| $500 | $450.00 | $400.00 | $350.00 | $250.00 |
Need to calculate a specific discount? Our Discount Calculator finds the sale price and exact savings in one step.