How to calculate percentages

By Haris Asif · Published October 4, 2026

Nearly every percentage problem is one of three questions. Once you can tell which one you are being asked, the arithmetic is short and the answer is easy to check.

What a percentage actually is

“Percent” means “per hundred”. Saying 35% is another way of writing 35/100 or 0.35. Converting between the three forms is the first skill to master: move the decimal point two places. 8% becomes 0.08, and 0.6 becomes 60%.

A percentage always describes a part of something. If you do not know what that something is (the whole), the number on its own tells you very little. “Sales rose 10%” means one thing for a corner shop and another for a multinational.

Question 1: What is X% of a number?

Convert the percentage to a decimal and multiply.

Example: a 12% service charge on a 60 bill. 60 × 12 ÷ 100 = 7.20. The total is 67.20.

In your head, split the percentage into pieces you can find easily. 10% is the number with the decimal moved one place left; 5% is half of that; 1% is the decimal moved two places. For 15% of 80: 10% is 8, 5% is 4, so the answer is 12.

Question 2: X is what percent of Y?

Divide the part by the whole, then multiply by 100.

Example: you answered 42 questions correctly out of 60. 42 ÷ 60 = 0.7, so you scored 70%.

The part does not have to be smaller than the whole. If this month’s expenses are 300 and last month’s were 200, this month is 300 ÷ 200 × 100 = 150% of last month.

Question 3: X is Y% of what number?

This works backwards from a part to find the whole.

Example: a 30 deposit is 15% of the full price. 30 × 100 ÷ 15 = 200, so the full price is 200.

A common mistake is to work out 15% of 30 and add it on. That treats 30 as the whole, which it is not.

Mistakes worth avoiding

• Using the wrong whole. “20% more than 50” and “20% of 50” are different questions. The first is 60, the second is 10.
• Adding percentages of different bases. 10% of one number plus 10% of another is not 10% of their sum unless the numbers share the same base.
• Treating a percentage point as a percent. Rates moving from 4% to 5% is a 1 percentage-point rise but a 25% relative increase. The percentage change guide covers this.
• Rounding too early. Keep extra decimals during the calculation and round only the final answer.

A quick check for any answer

Ask whether the answer is sensible. 20% of a number should be smaller than the number; 150% of it should be larger. If you find 15% of 80 and get 120, you probably multiplied by 1.5 instead of 0.15. A calculator is useful here, but a sanity check catches most slips.