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How to work out percentages on your phone
A discount on a sign, a tip on a bill, a price that went up since last month. Each one is a percentage, and each one is a single line on your phone calculator once you know which line to type. Five patterns cover the everyday cases.
Five ways to calculate percentages, each with a worked example, then calculator apps. The numbers in the examples are only arithmetic. The percent key works differently in different calculator apps, so the multiplications below give the same answer anywhere.
One multiplication for discounts
To take 15% off, multiply by 0.85. To add 15%, multiply by 1.15. It is one step instead of two and avoids the confusing percent key. Compare calculators in calculator apps.
Percent change
Subtract the old number from the new one, divide by the old one and multiply by 100. A negative result means a drop. This is how you compare price changes when tracking deals; see how to track price drops before you buy.
Taking out an included percentage
If a total already includes a percentage, divide by one plus that percentage. Do not subtract the percentage from the total. That gives the wrong answer.
Check with a rough guess
10% is the number with the decimal point moved one place left. Use it to sanity-check any answer. For help with homework, see how to use a math solver app to learn. For a household budget, see how to make a budget.
| Question | What you type | Example |
|---|---|---|
| A percent of a number | number × percent ÷ 100 | 15% of 80 = 80 × 15 ÷ 100 = 12 |
| Add a percent | number × (1 + percent ÷ 100) | 80 plus 15% = 80 × 1.15 = 92 |
| Take off a percent | number × (1 − percent ÷ 100) | 80 minus 15% = 80 × 0.85 = 68 |
| Percent change | (new − old) ÷ old × 100 | from 80 to 92 = 12 ÷ 80 × 100 = 15% |
| Remove an included percent | total ÷ (1 + percent ÷ 100) | 92 with 15% included = 92 ÷ 1.15 = 80 |
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100.
How do I take a discount off a price?
Multiply the price by one minus the discount as a decimal, for example 0.85 for 15% off.
How do I work out percentage change?
New minus old, divided by old, times 100.