In short: To find a percentage of an amount, turn the percentage into a fraction of 100 (or a decimal) and multiply it by the amount. Without a calculator, build the answer from easy parts: 10% is the amount divided by 10, 1% is divided by 100, and 5% is half of 10%. Add or subtract these chunks to reach any percentage you need.
Knowing how to find a percentage of an amount is one of the most useful skills in GCSE Foundation maths. It turns up in money questions, sale prices, test scores and data tables, so getting it secure pays off across the whole paper. The good news is that there is one reliable method that always works, plus a few quick tricks for the non-calculator paper. This guide walks through both, with a worked example you can follow step by step.
The reliable method
The core idea never changes: a percentage is just a number out of 100. Follow these steps.
- Write the percentage as a fraction of 100 or a decimal. For example, 35% becomes 35/100, which is the same as 0.35.
- Multiply that by the amount. "Of" in maths means multiply, so "35% of £240" means 0.35 × 240.
- Work out the multiplication. With a calculator this is one button press. Without one, use the chunking method below.
- Write the answer with the correct units, such as pounds, marks or kilograms.
For the non-calculator paper, build the answer from easy building blocks instead of multiplying directly:
- 10% of any amount is that amount divided by 10.
- 1% is the amount divided by 100.
- 5% is half of 10%.
- 50% is half the amount; 25% is a quarter.
Add or subtract these chunks to make the percentage you want. For example, 35% = 10% + 10% + 10% + 5%.
A worked example
A jacket costs £240. In a sale it has 35% taken off the price as a discount. How much money is taken off?
We need 35% of £240, without a calculator.
Start with the building blocks:
- 10% of £240 = 240 ÷ 10 = £24
- 5% of £240 = half of £24 = £12
Now build 35% from those chunks:
- 30% = three lots of 10% = 3 × £24 = £72
- 35% = 30% + 5% = £72 + £12 = £84
So £84 is taken off the price. (As a quick check, 0.35 × 240 = 84 on a calculator, which matches.)
This works because every percentage can be assembled from the simple 10%, 5% and 1% pieces. Once you can find 10% in your head, you can reach any percentage by adding a few easy parts together.
Common mistakes to avoid
- Trap: forgetting that "of" means multiply. Some students add the percentage to the amount instead. Fix: read "35% of £240" as "35% × £240" every time.
- Trap: dividing by 10 incorrectly. Writing 10% of £240 as £2.40 instead of £24. Fix: dividing by 10 moves the digits one place, so 240 becomes 24.0, which is £24.
- Trap: confusing the discount with the new price. The question asks how much is taken off, not what you pay. Fix: underline exactly what the question wants before you start.
- Trap: mixing up percentage of an amount with percentage change. Finding 35% of £240 is different from working out a percentage increase or decrease across stages. Fix: if a question chains several changes together, see [percentage change explained](/misconceptions/percent-change-family).
Frequently asked questions
How do you work out a percentage of a number without a calculator? Break the percentage into easy chunks. Find 10% by dividing the number by 10, find 1% by dividing by 100, and find 5% by halving the 10% value. Then add or subtract these chunks to build the percentage you need. For example, 15% is 10% plus 5%.
How do you find a percentage of an amount with a calculator? Convert the percentage to a decimal by dividing it by 100, then multiply by the amount. For 20% of £80, type 0.2 × 80 to get £16. You can also use the percent button if your calculator has one.
What is 15% of 200? 10% of 200 is 20, and 5% of 200 is half of that, which is 10. Adding them gives 20 + 10 = 30. So 15% of 200 is 30.
How do I turn a percentage into a decimal? Divide the percentage by 100, which moves the digits two places to the right. So 35% becomes 0.35, 7% becomes 0.07, and 120% becomes 1.2.
Why is finding 10% so useful? Because 10% is just the amount divided by 10, it is the quickest chunk to find in your head. Almost every other percentage can be built from 10% by multiplying, halving or adding, so it is the foundation of mental percentage work.
Practise this
Find out which mistakes cost marks — [take the free diagnostic](/diagnostic). Related: [percentage change explained](/misconceptions/percent-change-family).