Percentage Calculator

How to use the percentage calculator

This tool groups the four percentage questions people ask most often. Each box works on its own and updates as you type, so you can fill in only the one you need.

1. Percentage of a number: type the percentage and the number, for example “What is 25 % of 80?”. The answer (20) appears below.
2. Share as a percentage: type a part and a whole, for example “20 is what percent of 80?”, to get 25 %.
3. Percentage change: type an initial value and a final value, for example “From 50 to 65”. The result is signed (+30 %) and labelled as an increase or a decrease.
4. Increase or decrease by a percentage: choose Increase or Decrease, then type the starting value and the percentage, for example “Increase 80 by 15 %” gives 92.

Under each result, the calculation is written out step by step so you can check it or copy the method into your homework or spreadsheet. Use Clear to empty every box at once. You can type decimals with a point or a comma (12.5 or 12,5).

The formulas

A percentage is simply a fraction of 100: 25 % means 25/100 = 0.25. Every calculation in this tool comes from that idea.

• X % of Y = X / 100 × Y. Example: 25 % of 80 = 0.25 × 80 = 20.
• X is what percent of Y = X / Y × 100. Example: 20 / 80 = 0.25, so 25 %.
• Percentage change from A to B = (B − A) / |A| × 100. Example: from 50 to 65, the difference is 15, and 15 / 50 = 0.3, so +30 %.
• Increase Y by X % = Y × (1 + X / 100). Decrease Y by X % = Y × (1 − X / 100). Example: 80 × 1.15 = 92.

Dividing by the absolute value |A| in the percentage change formula keeps the sign meaningful when the starting value is negative: going from −10 to −5 is shown as +50 %, because the value went up.

Exact results

The calculations use exact fractions rather than binary floating point, so 0.1 % of 0.3 gives exactly 0.0003 and never a long tail such as 0.00030000000000000003. When a result has more than six decimal places, it is rounded for display and marked with “≈”, for example 1 is ≈ 33.333333 % of 3.

Common mistakes with percentages

• Increases and decreases are not symmetrical. A price that rises by 20 % and then falls by 20 % does not return to its starting point: 100 → 120 → 96. To undo a 20 % increase you need a decrease of about 16.67 %.
• Percentage points are not percentages. If an interest rate moves from 2 % to 3 %, it rose by 1 percentage point, but by 50 % in relative terms. Use Percentage change for the relative figure.
• Choose the right reference. In “X is what percent of Y”, Y is the whole. Swapping the two numbers gives a different answer (20 of 80 is 25 %, 80 of 20 is 400 %).
• Removing a tax. To find the price before a 15 % tax, divide by 1.15 rather than subtracting 15 %.