This percentage calculator solves the four everyday percent problems in one place: P% of a number, what percent one number is of another, the whole when you know a part and its percent, and the percentage difference between two values. Every answer comes with the working, so you can check your homework or copy the method into your notebook.
How to use the percentage calculator
- Pick the question you need to answer: P% of X, X is ?% of Y, X is P% of ? or Difference.
- Type the two numbers. Decimals and negative numbers work, and a comma is accepted as the decimal separator.
- Choose how many decimal places to show (0 to 6, default 2).
- Read the result and the working underneath. It updates as you type, no button needed.
Percentage formulas
A percent is a number out of 100, so 15% means 15/100 = 0.15. All four modes follow from that:
| Question | Formula | Example |
|---|---|---|
| What is P% of X? | P ÷ 100 × X | 15% of 80 = 0.15 × 80 = 12 |
| X is what percent of Y? | X ÷ Y × 100 | 30 of 120 = 0.25 × 100 = 25% |
| X is P% of what? | X ÷ (P ÷ 100) | 45 is 9% of 45 ÷ 0.09 = 500 |
| Percentage difference of A and B | |A − B| ÷ ((|A| + |B|) ÷ 2) × 100 | 10 and 6: 4 ÷ 8 × 100 = 50% |
The percentage difference divides the gap by the average of the two values, so the order doesn’t matter. When both values are 0, the difference is defined as 0%.
What percent did I get on my test?
This is the “X is what percent of Y?” question. Points earned go in X, points possible in Y:
- 42 out of 50 → 42 ÷ 50 × 100 = 84%
- 27 out of 30 → 27 ÷ 30 × 100 = 90%
- 1 out of 3 → 33.333…%, shown as 33.33% with 2 decimal places or 33.3333% with 4
To turn the percent into a letter grade, or to combine several tests with different weights, use the grade calculator.
Percentage difference vs. percentage change
The two are easy to mix up. Percentage difference has no direction: 6 and 10 differ by 50% whichever comes first. Percentage change starts from an original value and divides by it, so going from 6 to 10 is +66.67% (4 ÷ 6), while going from 10 to 6 is −40% (4 ÷ 10). If your task says “increased”, “decreased” or “from … to …”, you need percentage change, not this Difference mode.
Common mistakes
- Dividing the wrong way round. “30 is what percent of 120” is 30 ÷ 120, not 120 ÷ 30. The whole always goes underneath.
- Forgetting to divide by 100. 15% of 80 is 0.15 × 80 = 12, not 15 × 80 = 1,200.
- Adding percents of different wholes. 10% off and then another 10% off is 19% in total, not 20%, because the second 10% is taken from a smaller amount.
- Confusing percent with percentage points. A score rising from 60% to 75% is up 15 percentage points, which is a 25% increase.
- Dividing by zero. “5 is what percent of 0” has no answer; the calculator tells you instead of showing a wrong number.