Percentage change expresses how much a value grew or shrank relative to where it started. It's the right tool whenever the starting size matters — a $10 rise means something very different on a $20 item than on a $2,000 one.
The formula
Subtract the original value from the new value, divide by the absolute value of the original, and multiply by 100. A positive result is an increase; a negative result is a decrease.
Because the original value is the denominator, it can't be zero — there's no baseline to measure growth against, so the calculator reports that case rather than a misleading number.
Change vs. percentage points
Percentage change is easy to confuse with percentage points. Going from 10% to 15% is a 5 percentage-point rise, but a 50% increase. This tool reports the relative change; keep the distinction in mind when the values are themselves percentages.
A worked example
If a price rises from 80 to 100, the change is (100 − 80) ÷ 80 = 25%. If it falls from 100 to 80, the change is (80 − 100) ÷ 100 = −20%. The two aren't mirror images: the same 20-unit move is a larger share of the smaller starting number.
This asymmetry is why a 25% gain only undoes a 20% loss — not the reverse. An investment that drops 50% needs a 100% gain just to get back to even, a fact that trips up a lot of intuition.
Common uses and pitfalls
Percentage change is everywhere — price tracking, test scores, and the statistics in the news. The classic trap is a starting value of zero: with no baseline to grow from, the change is mathematically undefined, which is why this calculator flags that case rather than showing a misleading number.
The other pitfall is direction. Swapping which value is the “original” flips the sign and changes the magnitude, so always be clear which number came first before you quote a percentage.