Percentage Calculator

The three percentage calculations

1. X% of a number: "What is 15% of 200?" → 30. Use it for discounts, tips, sales tax and commissions. The calculator also shows what's left after taking that percentage away.
2. X is what percent of Y: "30 is what percent of 200?" → 15%. Use it for test scores, progress toward a goal or market share.
3. X is Y% of what: "30 is 15% of what number?" → 200. Use it to recover the original total when you only know a part.

Percentage formulas

X% of Y = Y × X ÷ 100
X as a percent of Y = X ÷ Y × 100
Total = X ÷ (Y ÷ 100)

A percentage is a fraction out of 100. Converting it to a decimal (15% = 0.15) lets you multiply or divide by it directly.

Mental math shortcuts

• 10%: move the decimal point one place to the left. 10% of 350 is 35.
• 5% is half of 10%, 15% is 10% + 5%, and 20% is double 10%.
• X% of Y equals Y% of X: 8% of 50 is the same as 50% of 8, which is 4.
• 1%: divide by 100, then multiply by whatever percentage you need.

Common uses

Example

To find 15% of 200, multiply 200 × 0.15 = 30. If you have 30 and 200 and want the percentage, divide 30 ÷ 200 × 100 = 15%. And if you know 30 is 15% of some number, divide 30 ÷ 0.15 = 200.

1. 15% of 200 = 200 × 15 ÷ 100 = 30
2. 30 out of 200 = 30 ÷ 200 × 100 = 15%
3. 30 is 15% of 30 ÷ 0.15 = 200

Common mistakes

• Mixing up percentage points and percent change: going from 10% to 15% is a 5-point increase, but a 50% relative increase.
• Using the wrong base: a 20% discount followed by a 20% markup doesn't get you back to the start ($100 → $80 → $96).
• Forgetting to divide by 100 when converting to a decimal (multiplying by 15 instead of 0.15).