Percent Change Calculator

Mode 1 worked example: percent change

Starting value 80, final value 100. The formula divides by the absolute starting value, not the final one:

Percent Change = ((100 − 80) / |80|) × 100 = (20 / 80) × 100 = 25%

This is a 25% increase. Reverse it — starting at 100, ending at 80 — and the change is (−20/100)×100 = −20%, not −25%. Percent change is not symmetric: a 25% increase from 80 to 100 is undone by a 20% decrease from 100 back to 80, because the base of the percentage shifts.

Mode 2 worked example: find the starting value

If you know the final value was 100 after a 25% increase, solving for the starting value rearranges the formula:

Starting Value = Final Value / (1 + Percent/100) = 100 / 1.25 = 80

Mode 3 worked example: find the final value

Starting at 80 with a known 25% increase:

Final Value = Starting Value × (1 + Percent/100) = 80 × 1.25 = 100

Modes 2 and 3 are exact algebraic inverses of Mode 1 — plugging any two of the three numbers (starting, final, percent) into the right mode always reproduces the third.

Why a starting value of zero breaks the calculation

Percent change is defined relative to the starting value, so a starting value of 0 makes the denominator zero — there's no meaningful "percent" of nothing. The tool blocks this case rather than returning Infinity or NaN.