Finding a Percentage of a Number
Finding a percentage of a number is one of the most practical maths skills there is. It tells you exactly how much a part of something is — whether that is a discount, a test score, or a tax amount.
The “building block” method below — breaking a percentage into easy chunks of 10%, 5%, and 1% — is a form of mental-maths technique long used by merchants and traders before calculators existed, since finding 10% of a number (just move the decimal point one place) is almost instant, and every other common percentage can be built from multiples and fractions of it. This mental shortcut is still exactly how experienced shoppers estimate a discount or a tip in their head at the till, without reaching for a phone calculator.
The Core Formula
Percentage of a number = (Percentage / 100) times the Number.
Or: convert the percentage to a decimal, then multiply.
Simple Method – Decimal Multiplier
Convert the percentage to a decimal (divide by 100), then multiply by the number.
30 / 100 = 0.30. 0.30 times 200 = 60.
15 / 100 = 0.15. 0.15 times 80 = 12.
7.5 / 100 = 0.075. 0.075 times 400 = 30.
Building Block Method
Break the percentage into easy parts using 10%, 1%, and 5%.
10% of 260 = 26. 30% = 3 times 26 = 78. 5% = half of 26 = 13. 35% = 78 + 13 = 91.
10% of 120 = 12.
5% of 120 = 6 (half of 10%).
2.5% of 120 = 3 (half of 5%).
17.5% = 10% + 5% + 2.5% = 12 + 6 + 3 = 21.
Handy Reference Table
| Percentage | Multiplier | Quick Method |
|---|---|---|
| 10% | 0.1 | Divide by 10 |
| 1% | 0.01 | Divide by 100 |
| 5% | 0.05 | Half of 10% |
| 25% | 0.25 | Divide by 4 |
| 50% | 0.5 | Divide by 2 |
| 75% | 0.75 | Three-quarters |
| 20% | 0.2 | Divide by 5 |
Real-Life Examples
- VAT at 20% on a £350 item: 0.2 times 350 = £70 tax.
- A 15% tip on a £46 meal: 0.15 times 46 = £6.90.
- A student needs 60% to pass. The test is out of 75: 0.6 times 75 = 45 marks needed.
Key Takeaways
- Convert the percentage to a decimal, then multiply by the amount.
- Use the building-block method (10%, 5%, 1%) for mental maths.
- The answer is always in the same units as the original number.
- Check: your answer should be smaller than the original when % is less than 100.