Percentage Calculator

What this tool does

Six percentage calculators in one page: a percentage of a number, what percent one number is of another, percentage increase or decrease, percentage difference, adding or subtracting a percentage, and reverse percentages. Results update as you type, with the formula shown using your own numbers.

Everything is calculated in your browser — nothing you type is sent anywhere.

How to use this tool

The formulas

• X% of Y = X / 100 × Y. So 20% of 150 = 0.20 × 150 = 30.
• X is what % of Y = X / Y × 100. So 30 is 12.5% of 240.
• Percentage change = (new − old) / old × 100. Going from 80 to 100 is +25%; from 100 to 80 is −20%.
• Percentage difference = |A − B| / ((A + B) / 2) × 100. For 80 and 100 that is 20 / 90 × 100 = 22.2%. This tool uses the average of the two values as the base; other conventions exist, such as dividing by the larger or the smaller value.
• Add or subtract a percentage = number × (1 + X/100) or number × (1 − X/100).
• Reverse percentage = final value / (1 + X/100) after an increase, or final value / (1 − X/100) after a decrease.

Change, difference and percentage points

These three get mixed up constantly. Percentage change has a direction: it measures how far a value moved from its starting point, so swapping the two numbers gives a different answer (+25% one way, −20% the other). Percentage difference has no direction. It compares two values on equal footing by dividing by their average, so 80 vs 100 and 100 vs 80 both give 22.2%. Use it when neither number is the “before”, for example two quotes from different suppliers.

Percentage points are the plain subtraction of two percentages. If a conversion rate rises from 4% to 5%, that is a 1 percentage-point gain, but a 25% relative increase. Both statements are true; they just answer different questions.

A common mistake: reversing a percentage change

A 50% rise followed by a 50% fall does not get you back to where you started. Take 100: add 50% and you have 150. Take 50% of 150 away and you have 75, a net loss of 25%. The second percentage is applied to a bigger number. To undo a +50% change you need a −33.3% change (100 / 150), and to undo −50% you need +100%. If you know the final value, use the Reverse % tab: 60 after a 20% increase was originally 60 / 1.20 = 50, not 60 less 20%.

Real-life examples

• Discounts: a $80 jacket at 25% off is $80 − $20 = $60 (Add / subtract %).
• VAT or sales tax: add 20% to 50 for 60. To strip tax from a 60 total, use Reverse % after an increase to get 50.
• Tips: 18% of a $64 bill is $11.52 (X% of Y).
• Grades: 42 out of 50 is 84% (X is what % of Y).
• Growth and interest: savings going from 2,000 to 2,150 is +7.5% (% increase / decrease).
• Marketing: a conversion rate moving from 2.0% to 2.5% is +0.5 points, or +25% relative.

Every figure on this page is elementary arithmetic that you can check by hand with the worked examples; no external sources are needed.

FAQ