Everyday Math · 5 minute read

How to calculate percentage increase or decrease

Measure how much a number changed relative to its original value using the standard percentage-change formula.

Quick answer: Subtract the old value from the new value, divide by the old value, and multiply by 100. A price rising from $80 to $100 increased by 25% because the $20 change is one quarter of the original $80.

Use the original value as the base

Percentage change compares the difference with where you started. The denominator is the old value, not the new value and not the average of the two.

A positive result means an increase. A negative result means a decrease. You can report the decrease as a positive percentage by using the size of the change and naming the direction separately.

Percentage change = (new value − old value) ÷ old value × 100

Percentage increase example

A monthly cost rises from $120 to $138. The change is $18. Divide $18 by the original $120 to get 0.15, then multiply by 100. The increase is 15%.

Percentage decrease example

A time falls from 50 minutes to 35 minutes. The decrease is 15 minutes. Divide 15 by the original 50 to get 0.30. The time decreased by 30%.

Do not divide by 35. That would compare the change with the ending value and answer a different question.

A rise and fall do not cancel each other

If a value increases by 20% and later decreases by 20%, it does not return to the starting point. Starting at 100, a 20% rise produces 120. A 20% decrease from 120 removes 24, leaving 96.

The percentages use different base values, which is why equal-looking changes can have different dollar or numeric effects.

Identify the original value first.
Find the numeric difference.
Divide by the original value.
Multiply by 100 and label the direction.

Frequently asked questions

What if the old value is zero?

The standard percentage-change formula is undefined because division by zero is impossible. Report the numeric change instead.

Is percentage change the same as percentage points?

No. A rate moving from 20% to 25% rises 5 percentage points but increases by 25% relative to the original rate.

Which value goes on the bottom?

Use the original or old value as the denominator.

Want help with the calculation?

Use the percentage calculator →

Published August 3, 2026 · Reviewed for clarity and source accuracy.