The Basic Formula

Percentage = (Part ÷ Whole) × 100

To find what X% of a number is: Number × X ÷ 100

Common Calculations

  1. 1

    Find X% of a number

    Multiply the number by the percentage, then divide by 100. Example: What is 20% of 250? 250 × 20 ÷ 100 = 50. On a calculator: enter 250 × 0.20 = 50. (Convert the percentage to a decimal by dividing by 100: 20% = 0.20, 15% = 0.15, 7.5% = 0.075.)

  2. 2

    What percentage is X of Y?

    Divide X by Y, then multiply by 100. Example: What percentage is 30 of 120? 30 ÷ 120 = 0.25 × 100 = 25%. So 30 is 25% of 120.

  3. 3

    Percentage increase or decrease

    ((New Value − Old Value) ÷ Old Value) × 100. Example: price went from $80 to $100. Increase = (100 − 80) ÷ 80 × 100 = 25% increase. If price dropped from $100 to $80: (80 − 100) ÷ 100 × 100 = −20% (a 20% decrease).

Quick Mental Maths Tricks

  • 10%: Move the decimal one place left. 10% of 350 = 35.
  • 5%: Find 10% then halve it. 5% of 350 = 17.5.
  • 20%: Find 10% then double it. 20% of 350 = 70.
  • 25%: Divide by 4. 25% of 350 = 87.5.
  • 50%: Divide by 2. 50% of 350 = 175.
  • 15%: Find 10% + half of 10%. 10% of 350 = 35, half = 17.5, total = 52.5.
  • 1%: Move decimal two places left. 1% of 350 = 3.5. Then multiply for any percentage.

Everyday Examples

  • GST (10% in Australia): Multiply price by 1.10 for GST-inclusive price. To find GST in a price: divide by 11.
  • Discount: 30% off $120 = $120 × 0.70 = $84. (Multiply by what remains: 100% − 30% = 70% = 0.70.)
  • Pay rise: 4% raise on $75,000 = $75,000 × 0.04 = $3,000 raise. New salary = $78,000.
  • Interest: 5% annual interest on $10,000 = $10,000 × 0.05 = $500 per year.
On your phone calculatorMost phone calculators have a % button. To find 15% of 200: type 200 × 15 % = (answer: 30). To apply a 10% discount to $85: type 85 − 10 % = (answer: 76.50).

Frequently Asked Questions

((New − Old) ÷ Old) × 100. If sales grew from $50,000 to $65,000: ((65,000 − 50,000) ÷ 50,000) × 100 = 30% increase. Always divide by the original (old) value, not the new one. A negative result means a decrease.
Percentage points are the arithmetic difference between two percentages. If interest rates rise from 3% to 5%, that is a 2 percentage point increase — but a 67% increase in the rate itself (2 ÷ 3 × 100). The distinction matters in finance and statistics: a politician saying “unemployment fell 2%” is different from “fell 2 percentage points.”