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.