Percentages Show Up Everywhere
Sale prices, tax, test scores, interest rates, and pay raises are all percentages. Most people only need four calculations, and each one follows a simple pattern.
1. Finding a Percentage of a Number
To find 15% of 240, convert the percent to a decimal by dividing by 100, then multiply: 15 / 100 = 0.15, and 240 x 0.15 = 36.
2. Percent Change
To measure how much something went up or down, subtract the old value from the new value, divide by the old value, and multiply by 100.
- Old price: $80. New price: $100.
- Change: $100 - $80 = $20
- Percent change: $20 / $80 x 100 = 25% increase
Be careful with direction. A drop from $100 to $80 is a 20% decrease, not 25%, because the starting value is different.
3. Finding the Original Amount
Sometimes you know the result and the percentage, but not the starting number. Divide the result by the percentage in decimal form. If 30 is 20% of a number, the number is 30 / 0.20 = 150.
4. Discounts
A 25% discount on a $60 item takes off $60 x 0.25 = $15, so you pay $45. Stores often advertise a discount on top of an existing one, so work out each step separately rather than adding the percentages together.
Quick Check
If you are unsure of an answer, estimate. 10% of any number is the same number with the decimal point moved one place to the left, and you can build other percentages from that.
Try It Yourself
The Percentage Calculator handles all four of these calculations in one place.