Quick answer
A percentage is a number out of 100. This calculator has three modes: find A% of B, find what percent A is of B, or find the whole when A is a given percent of it.
All three modes rearrange one relationship: part = percent × whole ÷ 100. Choose the mode that matches the number you do not know.
Related tools: Percentage Change Calculator: Find the percentage increase or decrease from an old value to a new value. Percent Difference Calculator: Symmetric percent difference between two values. Percent Increase Calculator: Percentage increase between two values.
How to use the Percentage Calculator
- Choose what do you want to find?: A% of B, A is what % of B and A is B% of what number.
- Enter a.
- Enter b.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| What do you want to find? | Choose one: A% of B; A is what % of B; A is B% of what number | Starts at A% of B |
| A | A number | — |
| B | A number | — |
Formula and method
Formula
A% of B = A × B ÷ 100; A is (A ÷ B × 100)% of B; whole = A ÷ (B ÷ 100)
IEEE-754 double precision; shown to at most 6 decimal places.
Percent means per hundred. Each mode rearranges part = percent × whole ÷ 100.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: What do you want to find? = A% of B; A = 15; B = 200.
| Result | Value |
|---|---|
| 15% of 200 | 30 |
Example 2
Inputs: What do you want to find? = A is what % of B; A = 30; B = 200.
| Result | Value |
|---|---|
| 30 is what percent of 200 | 15 % |
Example 3
Inputs: What do you want to find? = A is B% of what number; A = 30; B = 15.
| Result | Value |
|---|---|
| 30 is 15% of | 200 |
The first line is the headline result (15% of 200). IEEE-754 double precision; shown to at most 6 decimal places.
The three modes
| Question | Formula | Example |
|---|---|---|
| What is A% of B? | A × B ÷ 100 | 15% of 200 = 30 |
| A is what % of B? | A ÷ B × 100 | 30 is 15% of 200 |
| A is B% of what? | A ÷ (B ÷ 100) | 30 is 15% of 200 |
Percentages of 200
Every figure in this table comes from running this calculator, so it matches what the form returns.
| Percent | Result |
|---|---|
| 5 | 10 |
| 15 | 30 |
| 25 | 50 |
| 50 | 100 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- B: entering "0" shows "B must not be zero." beside that field and no result.
- A: left empty shows "Enter a." beside that field and no result.
Choosing between the Percentage Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Percentage Change Calculator](/math/percentage-change-calculator/) | old value and new value | Percentage change |
| [Percent Difference Calculator](/math/percent-difference-calculator/) | value 1 and value 2 | Percent difference |
| [Percent Increase Calculator](/math/percent-increase-calculator/) | original value and new value | Percentage increase |
| [Percent Decrease Calculator](/math/percent-decrease-calculator/) | original value and new value | Percentage decrease |
| [Percentage of a Percentage Calculator](/math/percentage-of-a-percentage-calculator/) | x and y | Result |
Background: rounding and basic arithmetic
Rounding to a place looks at the digit just to its right: 0 to 4 rounds down and 5 to 9 rounds up (the half-up convention, so 2.5 becomes 3). Rounding 838.274 gives 800 to the nearest hundred, 840 to the nearest ten and 838 to the nearest one.
Some systems use banker's rounding (half to even), which sends 2.5 to 2 and 3.5 to 4; check which convention your task requires, and round only at the end of a multi-step calculation.
Assumptions and limitations
- B = 0 is invalid for “what %”
- B = 0 is invalid for “whole” (would need to divide by 0%)
- Negative values are allowed
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for b and a to confirm this.
Frequently asked questions
What does the Percentage Calculator calculate?
Calculate a percentage of a number, what percent one number is of another, or the whole from a part. It returns 15% of 200.
What formula does the Percentage Calculator use?
A% of B = A × B ÷ 100; A is (A ÷ B × 100)% of B; whole = A ÷ (B ÷ 100). Percent means per hundred.
What do I need to enter in the percentage Calculator?
You enter What do you want to find?, A and B.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. B = 0 is invalid for “what %”.
What are the limits of this calculator?
B = 0 is invalid for “what %”. B = 0 is invalid for “whole” (would need to divide by 0%). Negative values are allowed.
What does the “What do you want to find?” option change?
It sets how the percentage is worked out. The choices are A% of B, A is what % of B and A is B% of what number.
What is an example of a percentage result?
With what do you want to find? A% of B, a 15 and b 200, the 15% of 200 is 30.
Related calculators
- Percentage Change Calculator — Find the percentage increase or decrease from an old value to a new value.
- Percent Difference Calculator — Symmetric percent difference between two values.
- Percent Increase Calculator — Percentage increase between two values.
- Percent Decrease Calculator — Percentage decrease between two values.
- Percentage of a Percentage Calculator — Find X% of Y%, optionally applied to a base amount.
- Ratio Percentage Calculator — Percentage share of each part of a ratio.
- Reverse Percentage Calculator — Work backwards from a final amount to the original value before a percentage increase or decrease.
- Fraction Calculator — Add, subtract, multiply and divide fractions with simplified and mixed-number results.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.