Quick answer
In a ratio A : B, A's share is A ÷ (A + B) × 100 and B's share is B ÷ (A + B) × 100. For 3 : 4, A is 42.857% and B is 57.143% of the whole.
Enter the two parts of the ratio. The calculator returns each part as a percentage of the total.
Related tools: Ratio Calculator: Simplify a ratio of two whole numbers. Percentage Calculator: Calculate a percentage of a number, what percent one number is of another, or the whole from a part. Percentage of a Percentage Calculator: Find X% of Y%, optionally applied to a base amount.
How to use the Ratio Percentage Calculator
- Enter part A (0 or more).
- Enter part B (0 or more).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Part A | A number | ≥ 0 |
| Part B | A number | ≥ 0 |
Formula and method
Formula
share of A = A ÷ (A + B) × 100
IEEE-754 double precision; shown to at most 6 decimal places.
Each part divided by the total.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Part A = 3; Part B = 4.
| Result | Value |
|---|---|
| Part A | 42.857143 % |
| Part B | 57.142857% |
Example 2
Inputs: Part A = 1; Part B = 1.
| Result | Value |
|---|---|
| Part A | 50 % |
| Part B | 50% |
The first line is the headline result (part A). Below it the ratio Percentage Calculator lists part B, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
Worked example
The total is 3 + 4 = 7. A: 3 ÷ 7 = 0.42857 = 42.857%. B: 4 ÷ 7 = 0.57143 = 57.143%. The shares add to 100%.
Use cases
Split costs, mix paint or dilute concentrate in a given ratio: a 1:3 mix is 25% concentrate. Share a $700 profit in the ratio 3 : 4 as $300 and $400. The total must be positive.
How the result changes with part A
Holding every other input at the first worked example, this table changes only part A. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Part A | Part A | Part B |
|---|---|---|
| 1.5 | 27.272727 % | 72.727273% |
| 3 | 42.857143 % | 57.142857% |
| 6 | 60 % | 40% |
| 12 | 75 % | 25% |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Part B: entering "0" shows "The parts must not both be 0." beside that field and no result.
Choosing between the Ratio 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 |
|---|---|---|
| [Ratio Calculator](/math/ratio-calculator/) | first number and second number | Simplified ratio |
| [Percentage Calculator](/math/percentage-calculator/) | what do you want to find?, a and b | 15% of 200 |
| [Percentage of a Percentage Calculator](/math/percentage-of-a-percentage-calculator/) | x and y | Result |
| [Percentage Change Calculator](/math/percentage-change-calculator/) | old value and new value | Percentage change |
| [Percent to Ratio Calculator](/math/percent-to-ratio-calculator/) | percent | Ratio |
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
- Total must be positive
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 part B to confirm this.
Frequently asked questions
What percentage is a 1:1 ratio?
A 1:1 ratio splits the total equally: 50% each.
How do I split $100 in 2:3?
Add the parts (2 + 3 = 5): $100 × 2/5 = $40 and $100 × 3/5 = $60.
Does it work with three parts?
Divide each part by the total of all three.
What does the Ratio Percentage Calculator calculate?
Percentage share of each part of a ratio. It returns Part A and Part B.
What formula does the Ratio Percentage Calculator use?
share of A = A ÷ (A + B) × 100. Each part divided by the total.
What do I need to enter in the ratio Percentage Calculator?
You enter Part A and Part B.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Total must be positive.
What is an example of a ratio Percentage result?
With part A 3 and part B 4, the part A is 42.857143 %.
Related calculators
- Ratio Calculator — Simplify a ratio of two whole numbers.
- Percentage Calculator — Calculate a percentage of a number, what percent one number is of another, or the whole from a part.
- Percentage of a Percentage Calculator — Find X% of Y%, optionally applied to a base amount.
- Percentage Change Calculator — Find the percentage increase or decrease from an old value to a new value.
- Percent to Ratio Calculator — Express a percentage as a part-to-whole ratio.
- Ratio to Decimal Calculator — Convert a ratio a:b to a decimal.
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.