Calculate a percentage of a number

Percentages: a proportion out of 100

A percentage is a fraction with a denominator of 100. Writing 20% means 20 out of 100, or 20/100 = 0.2.

To calculate p% of a number N, we use the formula:

p% of N = (p × N) / 100

Example: calculate 20% of 150.

20% of 150 = (20 × 150) / 100 = 3000 / 100 = 30.

Percentages to know off by heart

Percentage	Fraction	Quick calculation
50%	1/2	divide by 2
25%	1/4	divide by 4
10%	1/10	divide by 10
1%	1/100	divide by 100

Quick example: 25% of 80 = 80/4 = 20.

Finding a percentage from a part

If you know a part and a total, you calculate: percentage = (part / total) × 100.

Example: in a class of 25 pupils, 5 are absent. The percentage of absentees is (5/25) × 100 = 20%.

Common pitfall

Don’t forget to divide by 100! Many pupils simply calculate p × N and forget the final step of division, which results in a figure that is far too high.