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Percentage calculator

Three percentage questions on one page, answered as you type.

  • Runs in your browser
  • No sign-up

Then you can

How to use the percentage calculator

  1. Choose what you want to find: X% of Y, X is what % of Y, or % change from X to Y.
  2. Type your two numbers. The answer updates as you type.
  3. Read the big result, then the formula and extra lines below it, such as the amount added or removed.

The page address keeps your inputs, so you can copy the link to share a calculation.

The three formulas

Percent of a number: result = X ÷ 100 × Y. So 15% of 200 is 0.15 × 200 = 30. The tool also shows Y plus and minus that amount: 230 and 170.

What percent: result = X ÷ Y × 100. If you scored 42 out of 60, then 42 ÷ 60 × 100 = 70%.

Percentage change: result = (new − old) ÷ old × 100. A price that moves from 80 to 100 rose by 25%, while a move from 100 back to 80 is a fall of 20%. The two are not equal because each percent is measured against a different starting value.

Worked example

A jacket costs 3,200 and is marked down to 2,400. What is the discount?

  • Mode: % change from X to Y, with X = 3,200 and Y = 2,400.
  • (2,400 − 3,200) ÷ 3,200 × 100 = −25%.
  • The price fell by 25%, which is a saving of 800.

To see what 25% of 3,200 is directly, switch to X% of Y: 0.25 × 3,200 = 800.

Common mistakes

  • Percent points versus percent. Going from 10% to 12% is a rise of 2 percentage points, but a 20% increase in the rate.
  • Stacking percentages. Two discounts of 20% and 10% do not make 30%. Use the discount calculator to stack them correctly.
  • Wrong base. The base for change is always the starting value, never the end value.

Prices with tax are easier in the GST calculator, and growth over many years belongs in the compound interest calculator. Everything is calculated in your browser, and nothing you type is sent anywhere.

Questions

How do I calculate a percentage of a number?

Divide the percentage by 100 and multiply by the number. For 15% of 200 that is 0.15 × 200 = 30.

How do I find what percent one number is of another?

Divide the part by the whole and multiply by 100. 30 is 15% of 200 because 30 ÷ 200 × 100 = 15.

How is percentage change calculated?

Subtract the old value from the new one, divide by the old value and multiply by 100. Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = +25%.

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

Because the second percentage applies to a bigger number. 100 rises to 150, then falls 50% to 75, which is 25% below where you began.

Can I use negative numbers or decimals?

Yes. Decimals such as 2.5% work, and negative values give a negative result. A starting value of 0 cannot be used for percentage change.

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