Percentage change measures how much a value moved relative to where it started. The old value is always the base — that single rule prevents nearly every percent-change mistake.
How to calculate percentage change
Subtract the old value from the new value, divide by the old value, and multiply by 100. From 200 to 250: (250 − 200) ÷ 200 × 100 = +25%. A negative result is a decrease: from 250 back down to 200 is (200 − 250) ÷ 250 × 100 = −20%. Notice those aren't mirror images — the +25% up and −20% down describe the same gap measured from different bases.
Change vs difference — which one do you need?
Percentage change is directional: prices over time, this month vs last month, before vs after. Percentage difference is for comparing two values where neither is the starting point — say, the price of the same laptop at two shops. It divides by the average of the two values, so it's symmetric: comparing A to B gives the same answer as B to A. This calculator shows both so you can pick the honest one.
Worked examples
A stock rises from $50 to $150: +200% change. Website traffic drops from 12,000 to 9,000 visits: −25%. Two quotes for the same job, $840 and $920: percentage difference = 80 ÷ 880 × 100 ≈ 9.1%. For chained percent math (a rise then a fall), remember each step re-bases — try it in the percentage calculator.
Common pitfalls
Don't average percentage changes across periods (a +50% year then a −50% year is a net −25%, not zero). Don't call a move from 10% to 12% a "2% increase" — it's 2 percentage points, which is a 20% relative increase. And a −100% change means the value hit zero; anything "more negative" than that is impossible for quantities that can't go below zero.
For information only.