Percentage change measures how much a value has grown or shrunk relative to where it started. The formula is ((new value − old value) ÷ old value) × 100. The result is positive for an increase and negative for a decrease — the sign matters and is worth keeping rather than dropping, since "a change of −25%" and "a change of 25%" describe opposite situations.
Where this differs from a plain percentage calculation (see the Percentage Calculator) is the base: percentage change always divides by the original value, never the new one, and never an average of the two. If a stock price goes from $80 to $100, the change is ((100 − 80) ÷ 80) × 100 = 25%. If it then drops back from $100 to $80, the change is ((80 − 100) ÷ 100) × 100 = −20%, not −25% — because the second calculation's base is $100, not $80. That asymmetry is exactly why "up 25% then down 25%" does not return you to the starting value, and it's the single most common source of confusion around percentage change.
Worked example one: a company's headcount grows from 40 employees to 52. ((52 − 40) ÷ 40) × 100 = 30% increase. Worked example two: a product's price drops from $75 to $60. ((60 − 75) ÷ 75) × 100 = −20%, a 20% decrease.
A second common mistake, beyond the base-value issue above, is confusing percentage change with percentage-point change when comparing two percentages directly. If a tax rate moves from 20% to 25%, that's a 5 percentage-point increase, but it's a (25 − 20) ÷ 20 × 100 = 25% relative increase — both descriptions are accurate, but they answer different questions, and news reporting sometimes blurs the two. This calculator computes relative percentage change (the 25% version), which is what's normally meant when someone asks "by what percentage did X change."
Percentage change is also the standard way financial and statistical data gets reported over time — year-over-year revenue growth, inflation rates, population growth, and stock returns are all percentage-change figures, which is why the sign convention matters so much: a headline reporting "inflation fell to 3%" versus "prices fell 3%" describe entirely different economic situations, and only careful attention to what the base value represents (last year's price level versus this year's rate of increase) tells them apart. When comparing percentage changes across multiple periods, note that they don't simply add — three consecutive years of 10% growth do not total 30% growth, because each year's 10% is calculated on an already-grown base; the real cumulative growth compounds to roughly 33.1% (1.10³ ≈ 1.331).
Percentage change can also apply to non-monetary quantities — a runner's mile time dropping from 9:00 to 8:15 is a change of ((8.25 − 9) ÷ 9) × 100 ≈ −8.3%, treating minutes as a decimal (8:15 = 8.25 minutes) before applying the same formula. Whenever the "old" and "new" values are naturally in different units or formats, converting them to a single consistent numeric form before applying the formula is a necessary first step that's easy to skip by accident.
For calculating a straightforward percentage of a number, or what percent one number is of another, use the Percentage Calculator instead — this tool is specifically for before-and-after comparisons. The How to Calculate Percentage Change guide walks through the formula with several more worked examples, including how to handle a change where the "old value" is negative or zero (a genuine edge case where percentage change is undefined, since you can't divide by zero) and how percentage change behaves when a value crosses from positive to negative, where the usual formula still applies but the result can look misleadingly large.