How do I calculate a percentage of a value, one value as a percentage of another, or percentage change?

The word percentage covers several different operations. The correct equation depends on whether the unknown is an amount, a share of a base, or a change between an old and new value.

Choosing the percentage question before the formula

Enter two values to answer three common percentage questions at once: what A percent of B equals, what percentage A represents of B, and the percentage change from A to B.

The calculator also shows the signed numerical difference. Percentage-of calculations use B as their base, while percentage change uses A as the starting value.

Several answers from the same two values

The default values are reused across the calculator’s outputs to show how the same two numbers answer different percentage questions.

A% of B50
A is this % of B12.5%
Change from A to B700%
Difference175

Reading each output by its denominator

Check the output label before reading the number. ‘A percent of B’ multiplies, ‘A is what percent of B’ divides, and change uses the original value as its reference.

Multiplication, comparison and change

A% of B = (A ÷ 100) × B; A as % of B = (A ÷ B) × 100Change from A to B = [(B − A) ÷ A] × 100; Difference = B − A

Direction and zero-base errors

Swapping the two values changes the denominator and therefore the answer. Percentage relationships are directional even when the absolute difference is unchanged.

  • Both values represent comparable quantities where a comparison is intended.
  • The denominator must be non-zero for division-based outputs.
  • Displayed rounding can hide small differences in intermediate values.

Calculations related to percentage

The following tools examine neighbouring parts of the same calculation without changing the inputs or assumptions used above.

Quick answers

Frequently asked questions

Which value is the starting value for percentage change?

Value A is the starting value and Value B is the ending value.

Why does reversing A and B change the percentage?

Percentages depend on the chosen base value, so reversing the values changes the denominator.

What happens when a base value is zero?

Calculations that would divide by zero return zero in this calculator.

Educational content only. This guide is not financial advice.