The three percentage formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 20% of 150 = 30 |
| X is what % of Y? | X ÷ Y × 100 | 30 of 150 = 20% |
| % change X → Y | (Y − X) ÷ X × 100 | 80 → 100 = +25% |
Everyday uses
Tips and discounts (“15% off £40”), test scores (“42 out of 50”), price changes (“up from $3.20 to $3.80”), commission, tax, and growth rates all come down to one of these three formulas.
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Frequently asked questions
How do I calculate a percentage of a number?
Multiply by the percentage ÷ 100: 20% of 150 = 150 × 0.20 = 30.
How do I find what percent one number is of another?
Divide and ×100: 30 ÷ 150 × 100 = 20%.
How do I calculate percentage increase/decrease?
(new − old) ÷ old × 100. 80 → 100 = 25% increase; a negative answer is a decrease.
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