Black Friday. The sign screams “60% OFF!” You grab the jacket, feeling like a genius. But was it really a deal, or was the “original price” inflated to make the discount look bigger?
Knowing how to calculate discounts is a superpower that saves real money. Here’s how to never fall for fake deals again.
The Basic Discount Formula
To find the sale price, use this simple formula: Sale Price = Original Price × (1 - Discount Rate).
For a $120 jacket at 25% off: $120 × (1 - 0.25) = $120 × 0.75 = $90.
A shortcut that makes mental math easier: for 10% off, just move the decimal point one place left. 10% of $120 is $12. For 20%, double that: $24. For 25%, find 25% as a quarter: $120 ÷ 4 = $30 off.
Stacking Discounts
When you have multiple discounts — like a 30% sale plus a 15% coupon — they don’t simply add up to 45%.
Instead, apply them sequentially. Start with $100 at 30% off = $70. Then 15% off $70 = $59.50. Your actual total discount is 40.5%, not 45%.
Order doesn’t matter mathematically (30% then 15% gives the same result as 15% then 30%), but some retailers apply store discounts before coupons, which can affect how loyalty points or cash-back is calculated.
Spotting Fake Deals
Inflated “original” prices. Some retailers raise prices before a sale, then “discount” them back to normal. Check price history tools like CamelCamelCamel for Amazon or Honey for other retailers.
“Up to X% off” language. This means the maximum discount on a few items. Average discounts are usually much lower.
BOGO math. “Buy one get one 50% off” sounds great, but you’re only saving 25% total. For two $40 items: $40 + $20 = $60, versus $80 at full price. That’s a 25% discount, not 50%.
Bundle “savings.” Compare the bundle price to buying items individually. Sometimes the “bundle deal” is the same or even more expensive than purchasing separately.
Unit Price: The True Comparison Tool
The unit price (price per ounce, per count, per ml) is the only honest way to compare products of different sizes.
A 12-pack of paper towels for $15.99 ($1.33 per roll) vs. a 6-pack for $7.49 ($1.25 per roll) — the smaller pack is actually cheaper per roll.
Most US grocery stores show unit prices on shelf labels, but not all retailers do. When they don’t, divide the total price by the quantity yourself.
The 10-Second Mental Math Tricks
10% off: Move decimal left. $85 → $8.50 off → $76.50.
20% off: Find 10%, double it. $85 → $8.50 × 2 = $17 off → $68.
25% off: Divide by 4. $84 ÷ 4 = $21 off → $63.
33% off: Divide by 3. $90 ÷ 3 = $30 off → $60.
50% off: Divide by 2. This one’s easy.
Calculate Any Discount
Our free Discount Calculator and Unit Price Calculator do all this math instantly. Enter any price and discount percentage to see your savings, or compare unit prices across different package sizes.