Percentage formulas
- X% of Y = X ÷ 100 × Y. 15% of 200 = 30.
- X is what % of Y = X ÷ Y × 100. 30 is 25% of 120.
- Percent change = (new − old) ÷ |old| × 100. From 80 to 100 is +25%.
- Increase by % = X × (1 + P/100). Decrease by % = X × (1 − P/100).
- Percent difference = |A − B| ÷ ((A + B) ÷ 2) × 100, used when neither value is the “original”.
A common trap
A 20% drop followed by a 20% rise doesn't get you back where you started: 100 → 80 → 96. Percent changes compound; they don't add.
Percentage points vs. percent
If a conversion rate moves from 4% to 5%, that's a rise of 1 percentage point but a 25% increase.