Measure relative change between two numbers: the percentage increase or decrease, the absolute difference, and the multiplier (e.g. 1.25× for a 25% rise). A drop shows as a negative percentage.
Percentage change is (new − old) / old × 100. Always divide by the old value — the starting point — which is also why a 50% loss needs a 100% gain to recover.
Frequently asked questions
From 80 to 100 is what percent increase?
25% — the change (20) divided by the original (80).
Why isn't a 50% drop undone by a 50% rise?
After a 50% drop you need a 100% rise to get back: 100 → 50 → 100.
Can the old value be zero?
Percentage change from zero is undefined — any change from 0 is infinitely many percent.
中文说明
衡量两个数之间的相对变化：增长或减少的百分比、绝对差值，以及倍数（如增长 25% 即 1.25 倍）。下降会显示为负百分比。
百分比变化 = (新值 − 旧值) ÷ 旧值 × 100。务必除以旧值（起点）——这也解释了为什么亏损 50% 需要上涨 100% 才能回本。
常见问题
从 80 涨到 100 是增长百分之几？
25%——变化量 20 除以原值 80。
为什么跌 50% 后涨 50% 回不了本？
跌 50% 后需要上涨 100% 才能复原：100 → 50 → 100。
旧值可以是 0 吗？
从 0 出发的百分比变化无定义——从 0 到任何数都是无穷大的百分比。