Calculation
Mathematical Formula For Calculating a Discount
Discount Amount = Original Price × (Discount% / 100)
Sale Price = Original Price − Discount Amount
Example Broken Down With Steps
A jacket originally costs $80 and is on sale for 25% off.
Step 1: Calculate the discount: 80 × (25 / 100) = 80 × 0.25 = 20
Step 2: Subtract from original: 80 − 20 = 60
Result: The sale price is $60 and you save $20.
Faster Method: Multiply by What You Keep
Instead of finding the discount and subtracting it in two steps, multiply the price by the complement of the discount in one step:
Sale Price = Original Price × (1 − Discount% / 100)
Using the same example:
$$\text{25% off means you pay 75%: } 80 \times 0.75 = 60$$
One step instead of two. Here are the most common discount complements:
DiscountYou payMultiply by10% off90%0.9015% off85%0.8520% off80%0.8025% off75%0.7530% off70%0.7040% off60%0.6050% off50%0.50
Stacked Discounts Do Not Add Up
When a store offers “20% off plus an extra 10% off,” the total discount is not 30%. Each discount is applied to the already-reduced price, so the combined saving is always less than the sum of the two percentages.
The Formula
Combined factor = (1 − a / 100) × (1 − b / 100)
Total discount = (1 − Combined factor) × 100
Example Broken Down With Steps
A $100 item is advertised as “20% off + extra 10% off”:
Step 1: Apply 20% off: 100 × 0.80 = 80
Step 2: Apply 10% off the reduced price: 80 × 0.90 = 72
Step 3: Total savings: 100 − 72 = 28
Result: The actual discount is 28%, not 30%.
More Examples
Stacked discountsActual total discount10% + 10%19% (not 20%)20% + 10%28% (not 30%)20% + 20%36% (not 40%)30% + 20%44% (not 50%)50% + 50%75% (not 100%)10% + 10% + 10%27.1% (not 30%)
The difference between the expected total and the actual total grows as the individual discount percentages increase.