Percentages and basic math

Percentage calculator

Calculate percentages, percentage change, increases and decreases with a clear result and formula.

Calculate percentages

Example: 20% of 100 = 20.

What the percentage calculator shows

The percentage calculator shows the result, formula and a short explanation for the selected calculation. It supports five common modes: percentage of a number, one number as a percentage of another, percentage change, increasing by a percentage and decreasing by a percentage.

How to use the calculator

Choose the calculation type, enter the two values and press Calculate. The labels change with the selected mode so you can see whether the calculator needs a percentage, a base number, a starting value or an ending value.

Common percentage calculations

  • Percentage of a number answers questions such as what 20% of 100 is.
  • One number as a percentage of another shows how large a part is compared with the whole.
  • Percentage change compares a starting value with an ending value.
  • Increase and decrease modes apply a percentage to one value.

Formula and calculation logic

  • Percentage of a number = percentage / 100 * number.
  • Part as percentage of whole = part / whole * 100.
  • Percentage change = (new value - old value) / old value * 100.
  • Increase by percentage = value * (1 + percentage / 100).
  • Decrease by percentage = value * (1 - percentage / 100).

Example calculations

  • 20% of 100 = 20.
  • 25 is 25% of 100.
  • The change from 100 to 125 = 25%.
  • 100 increased by 20% = 120.
  • 100 decreased by 20% = 80.

Limitations and rounding

The calculator performs a mathematical percentage calculation from the values you enter. It does not provide tax, accounting, legal or financial advice. Results are formatted for readability, so very long decimals may be rounded in the display.

Frequently asked questions

How do you calculate a percentage of a number?

Divide the percentage by 100 and multiply by the number. For example, 20% of 100 is 20.

What is the difference between percent and percentage points?

A percent describes a relative share out of one hundred. A percentage point describes the difference between two percentages, such as moving from 10% to 15%, which is 5 percentage points.

Why do some calculations fail when the comparison value is zero?

Part-as-percentage and percentage-change calculations divide by the comparison value. If that value is zero, there is no single meaningful result.

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