Percentage of an amount — what is X% of Y
How do you find a percentage of an amount?
"What is X% of Y" is the most common percentage question in everyday life — a discount in a shop, a tip in a restaurant, VAT on a bill. The formula is fixed, but there are also shortcuts that make mental calculation far quicker than most people think.
Worked examples, step by step
What is 15% of £200?
- The formula: (Y × X) ÷ 100
- Substitute: (200 × 15) ÷ 100
- 200 × 15 = 3000
- 3000 ÷ 100 = 30
- Answer: 15% of £200 is £30
What is 30% of 90?
- First find 10% of 90: move the decimal point — 9
- 30% is 3 times 10%
- 9 × 3 = 27
- Answer: 30% of 90 is 27
A builder quotes £180 plus VAT. VAT is 20%. What is the total bill?
- Find 20% of 180: 10% is 18, so 20% is 36
- Add the VAT to the price: 180 + 36 = 216
- Or in one step: 180 × 1.2 = 216
- Answer: the total bill is £216
What is 8% of 350?
- The formula: (Y × X) ÷ 100
- Substitute: (350 × 8) ÷ 100
- 350 × 8 = 2800
- 2800 ÷ 100 = 28
- Answer: 8% of 350 is 28
Now try it yourself
Frequently asked questions
What is the formula for a percentage of an amount?
Percentage of an amount = (amount × percentage) ÷ 100. For example 20% of 80: (80 × 20) ÷ 100 = 16.
How do you calculate percentages in your head without a calculator?
The quickest way: find 10% first (move the decimal point one place to the left), then multiply or divide to reach the percentage you want. 10% of 250 is 25, so 20% is 50, and 5% is half of 10%, that is 12.5.
How do you find 1% of a number?
Move the decimal point two places to the left. 1% of 350 is 3.5. From 1% it is easy to reach any other percentage by multiplying — 7% of 350 is 3.5 × 7 = 24.5.
Is there a way to check that the answer is sensible?
Yes — if the percentage is close to 50%, the answer should be close to half the original amount. If the percentage is small (such as 5% or 10%), the answer should be much smaller than the original amount. A sense check like that catches most substitution mistakes.
✍️ Written by the MathsUK teamChecked against the National Curriculum and GCSE specificationsLast updated: