How it works
Percentage change is always measured against the starting value, not the ending one — that's what makes a 25% increase and a 20% decrease able to describe the exact same pair of numbers moving in opposite directions (80 to 100 is +25%, but 100 back to 80 is −20%, not −25%).
absolute change = new value − old value
percentage change = (new value − old value) ÷ old value × 100
Worked example
A value that moves from 80 to 100.
- Absolute change: 100 − 80 = +20.
- Percentage change: 20 ÷ 80 × 100 = +25%.
Going from 80 to 100 is a 25% increase. Going the other way, from 100 back to 80, would be a 20% decrease — not the same number, because the base changed.
Common questions
Why isn't a 25% increase reversed by a 25% decrease?
Because each percentage is measured against a different base. A 25% increase from 80 is measured against 80 (giving 100); reversing it would need a decrease measured against 100, and 25% of 100 is 25, not 20 — so a 25% decrease from 100 actually lands at 75, not back at 80.
What if the old value is zero?
Percentage change is undefined when the starting value is zero, because you'd be dividing by zero — any increase from nothing is technically an infinite percentage gain. This calculator flags that case rather than showing a misleading number.