How to Calculate a Percentage

How to calculate a percentage: quick answer

For a percentage of a number, convert the percentage to a decimal and multiply by the number. For example, 15% of 200 is 0.15 × 200 = 30. For “what percent is A of B?”, divide A by B and multiply by 100.

Percentage formulas at a glance

• Find X% of Y: (X ÷ 100) × Y
• Find what percent A is of B: (A ÷ B) × 100
• Find percentage change: ((New − Old) ÷ Old) × 100

Choose the formula that matches the question. If you need a quick calculation of a percentage of a number, use the Percentage Calculator.

What does a percentage mean?

If you are asking how to calculate a percentage, start by expressing the relationship as a part of 100. A percentage expresses an amount as a part of 100. For example, 25% means 25 out of every 100, which is the same as 0.25 as a decimal.

How to calculate a percentage of a number

To calculate a percentage quickly, use the formula below or try the free online Percentage Calculator.

To find X percent of Y, divide X by 100 and multiply the result by Y.

Formula: X% of Y = (X ÷ 100) × Y

Example: 20% of 150 = (20 ÷ 100) × 150 = 0.20 × 150 = 30.

How to calculate what percentage one number is of another

If you want to know what percentage one value represents of another, divide the part by the whole and multiply by 100.

Formula: Percentage = (Part ÷ Whole) × 100

Example: 30 out of 120 is (30 ÷ 120) × 100 = 25%.

How to calculate percentage change

Percentage change compares an old value with a new value.

Formula: Percentage change = ((New − Old) ÷ Old) × 100

For example, if a value rises from 80 to 100, the change is 20. Divide 20 by the original 80 and multiply by 100: 25%.

For a decrease, the same formula produces a negative result when the new value is lower than the old value.

How to calculate 10%, 25%, and other common percentages

Some percentages are easy to calculate mentally. Ten percent is one tenth of a number, 50% is one half, and 25% is one quarter. For example, 25% of 200 is 50 because 200 ÷ 4 = 50.

You can combine simple percentages too. For example, 15% is 10% + 5%, so 15% of 200 is 20 + 10 = 30.

How to calculate a discount

For a discount, first calculate the discount amount using the percentage-of-a-number formula, then subtract it from the original price.

Example: 20% off $80 is $16 because 20% of 80 is 16. The sale price is therefore $80 − $16 = $64.

For a quick calculation, use ToolAlto's Discount Calculator. For more examples, see How to Calculate a Discount and Sale Price.

How to calculate a percentage backward

If you know a final value after a percentage increase or decrease and need to find the starting value, work backward from the final amount instead of applying the same percentage in reverse.

For a discount of d%, the original value is final value ÷ (1 − d ÷ 100). For example, a $64 sale price after 20% off comes from an original price of $64 ÷ 0.80 = $80.

For a practical discount calculation, use the Discount Calculator or read How to Calculate a Discount and Sale Price.

Percentage points vs. percentage change

These terms describe different calculations. If a rate moves from 20% to 25%, the difference is 5 percentage points. The relative increase is 25% because 5 ÷ 20 × 100 = 25%.

Common percentage calculation mistakes

• Forgetting to divide the percentage by 100.
• Confusing “20% of 150” with “20 is what percent of 150?”
• Using the new value instead of the original value when calculating percentage change.
• Confusing percentage points with percent change.
• Rounding too early during multiple calculations.

FAQ