Percentage calculator

Three calculators for the three questions: a percent of a number, one number as a percent of another, and the change between two numbers. Type and the answer updates.

The three formulas

• Percent of a number: X% of Y = Y × X / 100. 15% of 80 is 12.
• One number as a percent of another: X / Y × 100. 12 is 15% of 80.
• Percent change: (new − old) / old × 100. From 80 to 92 is a 15% increase. Note the divisor is always the old value, so a 50% drop followed by a 50% rise does not get you back to the start.
• Reverse a discount: original = price / (1 − discount). $68 after 15% off was $80.

Common mistakes

• Percentage points are not percent: going from 10% to 12% is 2 percentage points, but a 20% increase.
• Stacked discounts do not add: 20% off then 10% off is 28% off, not 30%.
• Markup and margin differ: a 25% markup on cost is a 20% margin on price.