Percentage formulas
A% of B = A × B ÷ 100
A as a % of B = A ÷ B × 100
Percentage change = (new − old) ÷ old × 100
Add A% = B × (1 + A ÷ 100)
A as a % of B = A ÷ B × 100
Percentage change = (new − old) ÷ old × 100
Add A% = B × (1 + A ÷ 100)
Worked examples
- 20% of 150 is 30.
- 20 is 13.33% of 150.
- A price going from 20 to 150 is a 650% increase.
- 150 plus 20% is 180, and 150 minus 20% is 120.
Adding or removing GST? Use the GST calculator: taking 10% off a GST-inclusive price does not give the price before GST.
Frequently asked questions
How do I work out a percentage of a number?
Multiply the number by the percentage and divide by 100. 15% of 80 is 80 × 15 ÷ 100 = 12.
How do I calculate percentage increase?
Take the new value minus the old value, divide by the old value and multiply by 100. From $50 to $60 is a 20% increase.
Is a 10% rise then a 10% fall back to the start?
No. 100 plus 10% is 110, and 110 minus 10% is 99.
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Last updated October 2026.