Percentage Calculator

Solve the three everyday percentage questions, with each step shown so you can check it or do it by hand next time.

15% of 80
12
  • Calculation: 15 ÷ 100 × 80 = 12

Three ways to use percentages

"Percent" means "per hundred", so 15% is 15/100, or 0.15. Almost every percentage question is one of three: finding a percent of a number (15% of 80), finding what percent one number is of another (12 out of 80), or measuring how much something changed (from 80 to 92).

The trick in each case is knowing which number is the whole. In "X is what percent of Y", Y is the whole; in a percent change, the starting value is.

Common mistakes

Percent changes aren't symmetric: a 50% fall followed by a 50% rise leaves you at 75% of where you started, because the rise is calculated on a smaller number. And percentage points aren't percentages — a rate going from 4% to 5% is a 1 percentage point rise, but a 25% increase.

The formulas

X% of Y = X ÷ 100 × YX as a percent of Y = X ÷ Y × 100Percent change = (new − old) ÷ |old| × 100

Examples

  1. 15% of 80 = 0.15 × 80 = 12.
  2. 12 is what percent of 80? 12 ÷ 80 × 100 = 15%.
  3. From 80 to 92: (92 − 80) ÷ 80 × 100 = 15% increase.

Frequently asked questions

How do I work out a percentage in my head?

Find 10% by moving the decimal point one place left, then scale: 10% of 80 is 8, so 5% is 4 and 15% is 12.

How do I add a percentage to a number?

Multiply by 1 plus the percentage as a decimal: adding 8% sales tax to $50 is 50 × 1.08 = $54.

How do I find the original price before a discount?

Divide the sale price by 1 minus the discount: $60 after 25% off was 60 ÷ 0.75 = $80.

Last reviewed for 2026. How we calculate.