How to Calculate Percentages

A percentage is just a way of expressing a part out of 100. Almost every percentage question is one of three types, and once you can spot which type you are looking at, the maths is easy.

Percent of a number

To find a percentage of a number, turn the percent into a decimal and multiply. What is 20% of 500? Multiply 500 by 0.20 to get 100.

What percent one number is of another

Divide the part by the whole and multiply by 100. If 30 out of 200 questions were correct, that is 30 / 200 x 100 = 15%.

Percent increase and decrease

Take the difference, divide by the original, and multiply by 100. A price rising from 200 to 260 is (260 - 200) / 200 x 100 = a 30% increase. The same idea works for a drop: from 260 back to 200 is a decrease of about 23%, not 30%, because the starting number is different.

The mistake almost everyone makes

A percentage increase and the matching decrease are not equal. Add 30% to a number and you have to remove about 23% to get back, not 30%. And a percentage point is not a percent: moving from 10% to 12% is 2 percentage points, but that is a 20% relative increase.

Quick mental tricks

• 10% of anything: move the decimal one place left. 10% of 250 is 25.
• 5% is half of 10%, and 20% is double it.
• 1% is two places left. 1% of 250 is 2.5.
• X% of Y equals Y% of X, so 8% of 50 is the same as 50% of 8, which is 4.

Let a calculator do it

The Percentage Calculator handles all three types in one place. For price changes, try the Percentage Increase Calculator or the Discount Calculator.