How to Calculate Percentages: 5 Everyday Formulas

1. What is X% of a number?

Formula: number × (percentage ÷ 100).

Example: What is 15% of 80? 80 × 0.15 = 12.

2. What percent is one number of another?

Formula: (part ÷ whole) × 100.

Example: You scored 42 out of 50. 42 ÷ 50 = 0.84, × 100 = 84%.

3. Percentage increase

Formula: ((new − old) ÷ old) × 100.

Example: Rent goes from $450 to $495 a week. (495 − 450) ÷ 450 = 0.1, × 100 = a 10% increase. The Percentage Increase Calculator shows the working.

4. Percentage decrease

Formula: ((old − new) ÷ old) × 100.

Example: A laptop drops from $1,200 to $900. (1200 − 900) ÷ 1200 = 0.25, so it is 25% cheaper. See the Percentage Decrease Calculator.

5. Adding or taking off a percentage

• Add X%: multiply by (1 + X ÷ 100). $200 plus 10% = 200 × 1.1 = $220.
• Take off X%: multiply by (1 − X ÷ 100). $200 with 30% off = 200 × 0.7 = $140.

The mistake almost everyone makes

A percentage increase followed by the same percentage decrease does not bring you back to the start. If a $100 share rises 50% to $150, then falls 50%, it ends at $75 — because the second 50% is taken from the larger number.

In the same way, removing tax from a price is not the same as subtracting the tax rate. To remove 10% GST from $110, divide by 1.1 (giving $100), not subtract 10% (which gives $99). The GST & VAT Calculator handles this for any rate.

Everyday examples

• Tipping: a 15% tip on a $64 bill is 64 × 0.15 = $9.60.
• Pay rise: a 4% rise on a $70,000 salary adds 70,000 × 0.04 = $2,800, giving $72,800.
• Test score: 27 out of 32 is 27 ÷ 32 × 100 = 84.4%.
• Savings goal: if you have saved $3,600 of a $12,000 target, you are 3,600 ÷ 12,000 × 100 = 30% of the way there.

Percentage vs. percentage points

If an interest rate rises from 4% to 5%, it has gone up 1 percentage point — but that is a 25% increase in the rate itself. News reports often mix these up, so it is worth knowing the difference.

Quick reference