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How to Calculate a Percentage (The 3 Questions, with Examples)

By the SnapCalculations team · Updated 2 Nov 2026

The short answer

Most percentage questions are one of three types. X% of Y: multiply Y by X and divide by 100. What percent is A of B: divide A by B and multiply by 100. Percentage change: (new − old) ÷ old × 100.

What “percent” actually means

Percent means “out of 100”. So 25% is 25 out of every 100, which is the same as the fraction 25/100 or the decimal 0.25. To turn any percentage into a decimal, divide it by 100: 7% is 0.07, 150% is 1.5. Once you see percentages as decimals, the maths gets much easier.

Almost every everyday percentage question is one of three types. Spot which one you have, and use the matching formula.

1. Finding a percentage of a number

X% of Y = Y × X ÷ 100

Use this for discounts, tips, tax and “what is 15% of 200?”

Example: 15% of 200

What is 15% of 200?

  1. Turn 15% into a decimal: 15 ÷ 100 = 0.15.
  2. Multiply: 200 × 0.15 = 30.

Answer: 30

Try it in the calculator

Quick mental trick: 10% is the number with the decimal point moved one place left (10% of 64 is 6.4). From there, 5% is half of that, and 20% is double. And X% of Y always equals Y% of X, so 8% of 50 is the same as 50% of 8, which is 4.

2. Finding what percent one number is of another

A as a % of B = A ÷ B × 100

Use this for test scores, shares of a total, and progress towards a goal

Example: a test score

You scored 42 out of 60 on a test. What percentage is that?

  1. Divide the part by the whole: 42 ÷ 60 = 0.7.
  2. Multiply by 100: 0.7 × 100 = 70.

Answer: 70%

Try it in the calculator

3. Finding the percentage change

% change = (new − old) ÷ old × 100

A positive answer is an increase; a negative answer is a decrease

Example: a price rise

A monthly subscription goes up from 80 to 100. What is the percentage increase?

  1. Change = 100 − 80 = 20.
  2. Divide by the old value: 20 ÷ 80 = 0.25.
  3. Multiply by 100: 25.

Answer: A 25% increase

Try it in the calculator

Example: a fall in value

A phone's resale value drops from 250 to 200. What is the percentage decrease?

  1. Change = 200 − 250 = −50.
  2. Divide by the old value: −50 ÷ 250 = −0.2.
  3. Multiply by 100: −20.

Answer: A 20% decrease

Try it in the calculator

Notice that the base matters. Going from 80 to 100 is a 25% rise, but going back from 100 to 80 is only a 20% fall, because you divide by a different starting number each time.

Reverse percentages: finding the original amount

Sometimes you know the price after a percentage was added or taken off, and you want the price before. The trick is to divide, not subtract.

Example: removing 20% VAT

A UK price is £120 including 20% VAT. What is the price before VAT?

  1. Including VAT, the price is 100% + 20% = 120% of the original, which is 1.2 as a decimal.
  2. Divide by that: £120 ÷ 1.2 = £100.
  3. Check: 20% of £100 is £20, and £100 + £20 = £120. ✓

Answer: £100 before VAT (the VAT was £20, not £24)

Example: the price before a sale

A jacket costs 68 after 15% off. What was the original price?

  1. After 15% off you pay 85% of the original, which is 0.85.
  2. Divide: 68 ÷ 0.85 = 80.

Answer: 80

Percent vs percentage points

If an interest rate rises from 4% to 5%, it has gone up by 1 percentage point, but by 25 percent (because 1 is a quarter of 4). News reports often mix these up. Use “percentage points” for the simple difference between two percentages, and “percent” for the relative change.

Common mistakes to avoid

  • Dividing by the wrong number in a percentage change. Always divide by the old (starting) value, not the new one.
  • Undoing a percentage by subtracting it. Taking 20% off £120 gives £96, not the original £100. Divide by 1.2 instead.
  • Adding percentages that apply one after another. 20% off and then a further 10% off is 28% off in total, not 30%, because the second discount applies to the already reduced price.
  • Mixing up percent and percentage points, as described above.
  • Forgetting to sense-check. 25% of 80 is 20; check by working out 20 ÷ 80 = 0.25. If an answer looks too big or too small, it probably is.

Do it in one click

Our percentage calculator answers all three question types side by side and shows the working for each, so you can check your own sums or skip straight to the answer.

Free, with every step shownOpen the Percentage Calculator

Frequently asked questions

How do I calculate a percentage on a phone calculator?

For 15% of 200, type 200 × 15 ÷ 100 = 30. For “what percent is 42 of 60”, type 42 ÷ 60 × 100 = 70. Using × 100 and ÷ 100 avoids the % button, which works differently on different calculators.

How do I add a percentage to a number?

Multiply by 1 plus the percentage as a decimal. To add 20% to 50, do 50 × 1.2 = 60. To take 20% off, multiply by 0.8 instead.

Can a percentage be more than 100%?

Yes. 150% of 40 is 60, and a value that triples has increased by 200%. Only a share of a whole (like a test score) is capped at 100%.

Is a 50% rise followed by a 50% fall back to the start?

No. 100 rises 50% to 150, then falls 50% to 75. Each percentage applies to a different starting value.