How to Calculate a Percentage

Type 1: What is X% of Y?

Turn the percent into a decimal (divide by 100) and multiply. Example: what is 15% of 200? 15 ÷ 100 = 0.15, and 0.15 × 200 = 30. A mental shortcut: find 1% first (200 ÷ 100 = 2), then multiply by the percent (2 × 15 = 30). This is the shape behind discounts, commissions and tips.

Type 2: X is what percent of Y?

Divide the part by the whole, then multiply by 100. Example: 30 is what percent of 200? 30 ÷ 200 = 0.15, and 0.15 × 100 = 15%. This is the shape behind exam percentages and completion rates.

Type 3: Percentage change from old to new

Find the difference, divide it by the old value, and multiply by 100. Example: a price rises from 80 to 92. The difference is 12; 12 ÷ 80 = 0.15; so the increase is 15%. The base is always the starting value — that is why going from 80 to 92 (+15%) and back from 92 to 80 (−13.04%) use different percentages.

Three traps to avoid

Adding stacked percentages. 20% off followed by 10% off is 28% off, not 30% — the second discount applies to the reduced price.

Percent vs percentage points. A rate moving from 3% to 5% rose 2 percentage points, which is a 66.67% increase in the rate itself.

Up then down. Increase a value by 25% and then decrease the result by 25% and you end 6.25% below where you started — the decrease is measured against the larger, increased value.

Every type has a dedicated tool on this site: the percentage calculator covers all three plus increase-by, decrease-by and percentage difference.