Percentage Calculator
Four percentage calculations in one tool: percent of a number, "X is what % of Y", percentage change, and percent difference.
How to use this calculator
1. Pick the mode
Four tabs cover the most common percentage questions: "X% of Y", "X is what % of Y", percentage change, and percent difference. Each has its own input layout.
2. Fill in the two numbers
Each mode needs exactly two inputs. The result and an equation showing the math both appear below.
3. Avoid common mistakes
Percentage change and percent difference are not the same. Change uses the original as the denominator (directional); difference uses the average of both (symmetric). Use change when one value comes first; difference when comparing two equally-weighted values.
Mental shortcut
For 1% of a number, move the decimal two places left. For 10%, one place left. Everything else is multiples of those — 5% is half of 10%, 25% is a quarter, and so on.
Percentage formulas
X% of Y
(X / 100) × Y. Example: 20% of 150 = 0.20 × 150 = 30.
X is what % of Y
(X / Y) × 100. Example: 30 of 150 → (30 / 150) × 100 = 20%.
Percentage change
((new − old) / old) × 100. A positive result means an increase; negative means a decrease.
Percent difference
|A − B| / ((A + B) / 2) × 100. This is symmetric — A vs B gives the same result as B vs A — and works for comparing two equally weighted values.