Percentage change tells you how much a value has grown or shrunk relative to its starting point. It's one of the most useful metrics in finance, business, science, and daily life — whenever you need to express the magnitude of a shift in meaningful, comparable terms.
The percentage change formula
A positive result means the value increased. A negative result means it decreased. The old value is always the reference — it goes in the denominator. This is what makes percentage change directional: it matters which value came first.
Worked examples
- Price increase: A product goes from $80 to $100. Change = ((100 − 80) ÷ 80) × 100 = (20 ÷ 80) × 100 = +25%.
- Stock drop: A stock falls from $150 to $120. Change = ((120 − 150) ÷ 150) × 100 = (−30 ÷ 150) × 100 = −20%.
- Revenue growth: Quarterly revenue goes from $1.2M to $1.5M. Change = ((1.5 − 1.2) ÷ 1.2) × 100 = +25%.
- Weight loss: Body weight drops from 200 lbs to 185 lbs. Change = ((185 − 200) ÷ 200) × 100 = −7.5%.
Percentage change vs. percentage difference
These are often confused but serve different purposes. Percentage change is directional — it measures how much something moved from a specific starting point. Percentage difference is symmetric — it compares two values using their average as the reference, with neither treated as "before" or "after." Use percentage change when there's a clear time sequence (last month vs. this month). Use percentage difference when comparing two independent measurements (Lab A's reading vs. Lab B's reading).
Real-world applications
- Investing: Track portfolio returns, compare year-over-year stock performance, or measure how much your savings have grown.
- Business: Report revenue growth, monitor customer churn rates, and benchmark KPIs against prior periods.
- Health: Measure weight loss progress, blood pressure changes, or cholesterol improvements over time.
- Shopping: Determine whether a "new lower price" is actually a meaningful reduction or just a few cents.
Common pitfalls
- Reversibility trap: A 50% increase followed by a 50% decrease does NOT return to the original. $100 + 50% = $150, then $150 − 50% = $75. The base changes with each calculation.
- Zero baseline: Percentage change is undefined when the old value is zero — you cannot divide by zero. Use absolute change instead.
- Small-base distortion: A jump from 2 to 6 is a 200% increase, which sounds dramatic but is only 4 units. Always pair percentage change with the actual magnitude for context.