Skip to the calculator

How-to guides

How to calculate percentage: every common case

Updated 2026-11-13

Quick answer

A percentage is a number out of 100. To find X% of Y, multiply Y by X and divide by 100: 15% of 2,400 is 15 ร— 2,400 รท 100 = 360. The other common cases are shown below.

Steps

  1. Turn the percentage into a decimal by dividing by 100 (15% becomes 0.15).
  2. To find the percentage of a number, multiply the number by that decimal.
  3. To find what percent one number is of another, divide the part by the whole and multiply by 100.
  4. To find a percent change, subtract the old value from the new, divide by the old value and multiply by 100.
  5. To add or take off a percentage, multiply by 1 plus or minus the decimal.

The five calculations

X% of Y: multiply. 20% of 80 is 0.2 ร— 80 = 16. What percent is X of Y: divide and multiply by 100. 30 out of 120 is 30 รท 120 ร— 100 = 25%.

Increase a number by a percentage: multiply by 1 + the decimal. Adding 18% to โ‚น500 is 500 ร— 1.18 = โ‚น590. Decrease it: multiply by 1 โˆ’ the decimal. 20% off 80 is 80 ร— 0.8 = 64. And to find the whole when you know a part, divide: if 360 is 15% of a number, the number is 360 รท 15 ร— 100 = 2,400.

Percentage change is not symmetric

From 80 to 100 is a 25% increase, because the change of 20 is 25% of 80. From 100 back to 80 is a 20% decrease, because the change of 20 is 20% of 100. The same gap is a different percentage depending on where you start.

This is why two discounts do not add up. โ‚น2,000 with 25% off is โ‚น1,500, and a further 10% off gives โ‚น1,350. That is 32.5% off in total, not 35%.

Percent versus percentage points

If an interest rate moves from 10% to 12%, it has risen by 2 percentage points. As a percentage change of the rate itself, it has risen by 20%. News reports mix these up, so check which one is meant.

Try it yourself

Common questions

How do I calculate 15% of a number?

Multiply the number by 0.15, or by 15 and divide by 100.

How do I add GST to a price?

Multiply by 1 plus the rate as a decimal. At 18%, multiply by 1.18.

How do I find the original price after a discount?

Divide the sale price by 1 minus the discount. A โ‚น350 price after 30% off was 350 รท 0.7 = โ‚น500.

Why is a 50% rise then a 50% fall not back to the start?

The fall is taken on the bigger number. 100 up 50% is 150; 150 down 50% is 75.

Also read: How EMI is calculated, with a worked example