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Percentage Calculator: Percent Of, Percent Change, Increase and Decrease

Type your numbers into X% of Y, what percent is X of Y, percentage change, increase or decrease by a percent, or reverse percentage, and see every answer instantly.

Calculator

X% of Y

%

30

X is what % of Y?

20%

Percentage change

25%

Increase

Increase / decrease a value by %

%

Increase220

Decrease180

Reverse percentage: X is Y% of what?

%

150

Percentage difference

40%

Measured against the average of both numbers, so it is the same whichever number comes first.

Estimates for planning only. Not tax or financial advice. Last verified: 2026-10-25

Percentage calculator: how it works

A percentage is a fraction of 100. Almost every percentage question is one of a few patterns. To find X% of Y, multiply Y by X and divide by 100. To find what percent X is of Y, divide X by Y and multiply by 100. To find the percentage change, subtract the old value from the new value, divide by the old value and multiply by 100.

The calculator covers those three plus increase by a percent, decrease by a percent and the reverse question (30 is 15% of what number?). Percentage change is always measured against the starting value, which is why a rise of 25% followed by a fall of 25% does not return to where you began.

Formula and assumptions

Formula

  • X% of Y = X ÷ 100 × Y
  • X is what % of Y = X ÷ Y × 100
  • % change = (new − old) ÷ |old| × 100

Assumptions

  • Percentage change is relative to the starting value.
  • Results are shown to up to 4 decimal places.

Worked example

Five common questions

  1. 15% of 240 = 15 ÷ 100 × 240 = 36.
  2. 45 is what percent of 60? 45 ÷ 60 × 100 = 75%.
  3. Change from 80 to 100: (100 − 80) ÷ 80 × 100 = +25%.
  4. Increase 200 by 12%: 200 × 1.12 = 224.
  5. 30 is 15% of what? 30 ÷ 15 × 100 = 200.

Result: 36; 75%; +25%; 224; 200.

Limitations

  • Percentage change is undefined when the starting value is zero, so the calculator shows no result in that case.
  • Results are shown to up to four decimal places and rounded for display.
  • Percentage points and percent are different: moving from 10% to 12% is +2 percentage points but a 20% relative increase.
  • The tool does not know the context, such as whether a percentage change is compounding over several periods.

Frequently asked questions

How do I calculate a percentage increase?

Subtract the old value from the new value, divide by the old value and multiply by 100. From 80 to 100 that is 20 ÷ 80 × 100 = 25%.

Why is a 25% rise followed by a 25% fall not back to the start?

The fall is measured on the higher value. 100 rises to 125, then falls 25% of 125 (31.25) to 93.75.

What is the difference between percent and percentage points?

Percentage points are the arithmetic difference between two percentages. Percent change is the relative difference. A rate going from 5% to 6% is up 1 percentage point and up 20%.

How do I find the original number before a percentage was added?

Divide the new number by one plus the percentage as a decimal. If a price is 224 after a 12% increase, the original was 224 ÷ 1.12 = 200.

Can a percentage be more than 100%?

Yes. 150% of 40 is 60. A percentage change over 100% means the value more than doubled.

Does it work with negative numbers?

You can enter negatives, and the change formula uses the absolute value of the starting number, but interpret results with care.

How do I calculate a percentage of a number?

Multiply the number by the percentage and divide by 100. For example, 15% of 80 is 80 × 15 ÷ 100 = 12. Use the X% of Y mode above to see the answer instantly.

How do I calculate percentage change between two numbers?

Subtract the old number from the new one, divide by the old number and multiply by 100. From 80 to 100 is a 25% increase; from 100 to 80 is a 20% decrease. The base is always the starting value, which is why the two changes are not equal.

Sources

Calculator data last verified: 2026-10-25. Estimates for planning only; not tax or financial advice.

What next?

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