Percentage of a Number Calculator
Work out what a percentage of a number comes to — tips, discounts, commission, VAT, a share of a total.
A percentage is a fraction of a hundred, so "X% of Y" is Y × X ÷ 100.
How the percentage of a number calculator works
A percentage is a fraction of a hundred, so "X% of Y" is Y × X ÷ 100. Multiplying by 15 and dividing by 100 is the same as multiplying by 0.15, which is the quicker way to do it on a calculator. For mental arithmetic, split the percentage into easy pieces: 10% is one decimal place, 5% is half of that, and 1% is two decimal places.
Formula: X% of Y = Y × X / 100; X as % of Y = X / Y × 100; change = (B − A) / A × 100
Worked examples
| Inputs | Answer | Note |
|---|---|---|
| 15% of 240 | 36 | 36 |
| 20% tip on 85 | 17 | 17 |
| 7.5% of 1,200 | 90 | 90 |
FAQFrequently asked questions
How do I calculate a percentage of a number?
Multiply by the percentage and divide by 100 — or just multiply by the decimal: 15% of 240 is 240 × 0.15 = 36.
How do I work out 10% in my head?
Move the decimal point one place left. 10% of 85 is 8.5, and 5% is half of that.
How do I calculate a tip?
Take 10%, then double it for 20% or add half for 15%.
What is 15% of 240?
36.
Does the order matter?
No — 15% of 240 equals 240% of 15. Both are 36, which is sometimes a useful shortcut.
Where these figures come from
- NIST Digital Library of Mathematical Functions — reference definitions for elementary and special functions
- Wolfram MathWorld — definitions and formulas for every topic on this page
- NIST/SEMATECH e-Handbook of Statistical Methods — the statistical formulas (mean, variance, z, confidence intervals, sample size)
- Common Core State Standards — Mathematics — the terms and methods taught in US schools
Last checked: September 2026. Formulas are fixed by mathematics and do not change with tax years or regulations.