Percentage of an Amount (No Calculator) — KS3 Maths

Every percentage question without a calculator starts in the same place: find 10% by dividing by 10. From that one number you can build any percentage you need — 30% is three of them, 5% is half of one, and 15% is a 10% plus a 5%.

How do you find a percentage in your head?

Ten percent is the anchor: dividing by 10 is quick, and every other percentage is built from it by multiplying, halving, or adding two pieces together.

Anything else is a combination of these rows.

Worked example

30% of 40

Find 10% first: $40 \div 10 = 4$. Thirty percent is three of those 10% blocks:

$30\% \text{ of } 40 = 3 \times 4 = 12$

Worked example

70% of 250

Ten percent of $250$ is $250 \div 10 = 25$. Seventy percent is seven blocks:

$70\% \text{ of } 250 = 7 \times 25 = 175$

Worked example

15% of 60 (10% + 5%)

Ten percent of $60$ is $6$. Five percent is half of ten percent, so half of $6$ is $3$. Fifteen percent is $10\% + 5\%$:

$15\% \text{ of } 60 = 6 + 3 = 9$

Where you'll use this next

• Percentage increase and decrease is this skill plus one more step: add it on, or take it off.
• Reverse percentages run the process backwards to find an original price.
• Tips, VAT, interest and discounts are all this calculation with a context attached.

Practice questions

Find 10% first, then build up. Then click to check.