The three key percentage formulas
- X% of Y: Multiply Y by X ÷ 100 — e.g., 20% of 250 = 50
- X is what % of Y: X ÷ Y × 100 — e.g., 50 ÷ 200 × 100 = 25%
- Percentage change: (New − Old) ÷ Old × 100 — e.g., 80 to 100 = 25% increase
Real-world examples
- 30% off $120 = $36 off = $84 to pay
- $18,000 tax on $80,000 income = 22.5% effective tax rate
- 72 out of 90 on a test = 80%
- Price went from $4.50 to $5.00 = 11.1% increase
- GST: add 10% to ex-GST price, or divide inc-GST price by 1.1 to get ex-GST
GST tip for Australians: To find the GST component of a price that already includes GST, divide by 11. For example, $110 inc GST ÷ 11 = $10 GST component.