Percentage Tricks
You do not always need a calculator to work out a percentage. With a handful of simple mental maths tricks, you can find 10%, 25%, 50%, 75% and many other percentages entirely in your head. These shortcuts are useful for shopping, tipping, estimating and non-calculator exam papers.
The core tricks at a glance
- 50% Divide by 2
- 25% Divide by 4 (or halve twice)
- 75% Find 50% + 25%
- 10% Divide by 10
- 20% Double 10%
- 5% Halve 10%
- 1% Divide by 100
The 10% trick
Ten per cent is one tenth of a number. To find 10%, move the decimal point one place to the left, which is the same as dividing by 10.
The 5% trick
Five per cent is half of 10%. Find 10% first, then halve the result.
5% of 80
10% = 8, then ÷ 2 = 4
5% of 340
10% = 34, then ÷ 2 = 17
5% of 260
10% = 26, then ÷ 2 = 13
The 1% trick
One per cent is one hundredth of a number. To find 1%, move the decimal point two places to the left, which is the same as dividing by 100.
The 20% trick
Twenty per cent is double 10%. Find 10% then double it.
20% of 80
10% = 8, then × 2 = 16
20% of 250
10% = 25, then × 2 = 50
20% of 95
10% = 9.5, then × 2 = 19
The 25% trick
Twenty-five per cent is one quarter. Divide the number by 4, or halve it twice.
The 50% trick
Fifty per cent is one half. Simply divide by 2.
The 75% trick
Seventy-five per cent is three quarters. The easiest approach is to find 50% and 25%, then add them together.
75% of 80
50% = 40, 25% = 20, total = 60
75% of 120
50% = 60, 25% = 30, total = 90
The commutative trick
Here is one of the most surprising percentage facts: X% of Y is always equal to Y% of X. This is because multiplication is commutative (the order does not matter).
X% of Y = Y% of X
This becomes very handy when one direction is easier to calculate mentally than the other.
What is 4% of 25?
Hard to calculate directly. But 25% of 4 = 4 ÷ 4 = 1
What is 8% of 50?
Flip it: 50% of 8 = 8 ÷ 2 = 4
What is 12% of 25?
Flip it: 25% of 12 = 12 ÷ 4 = 3
Building any percentage from parts
You can build up any percentage by combining the simpler ones you already know. Start with 10% and 1%, then add or subtract to reach the target.
15% = 10% + 5%
Find 10%, then add half of 10%
30% = 3 × 10%
Find 10% and multiply by 3
35% = 30% + 5%
Find 30%, then add 5%
17% = 10% + 5% + 2 × 1%
Build up from 10% + 5% + 1% + 1%
Worked examples using the tricks
Example 1: Find 15% of 220
10% of 220 = 22
5% of 220 = 22 ÷ 2 = 11
15% = 22 + 11 = 33
Example 2: Find 35% of 60
10% of 60 = 6
30% = 6 × 3 = 18
5% = 6 ÷ 2 = 3
35% = 18 + 3 = 21
Example 3: Find 17.5% of 80 (VAT-style calculation)
10% of 80 = 8
5% = 8 ÷ 2 = 4
2.5% = 4 ÷ 2 = 2
17.5% = 8 + 4 + 2 = 14
Example 4: Use the commutative trick to find 4% of 75
4% of 75 is awkward. Flip it: 75% of 4.
75% of 4 = 50% + 25% = 2 + 1 = 3
Example 5: Find 25% of £48
25% = one quarter
£48 ÷ 4 = £12
Example 6: Find 12% of 250 without a calculator
10% of 250 = 25
1% of 250 = 2.5
2% = 2.5 × 2 = 5
12% = 25 + 5 = 30
Try the calculators
Use these calculators to check your mental maths or to handle more complex percentage problems.