A ratio compares two parts to each other; a percentage compares one part to the whole. Converting between them means first adding the parts to get the size of the whole, then dividing each part by that total. A ratio of 3:2 has five parts in total, so the first side holds three fifths and the second holds two fifths.
The step people miss is that the denominator is the sum of the parts, not the other part. Reading 3:2 as "three out of two" gives 150%, which is the answer to a different question — that is the ratio expressed as a percentage of the second quantity, useful when comparing two separate totals rather than splitting one. Both figures appear below so you can pick the one your question actually asks for.
The formula
a- The first term of the ratio
b- The second term of the ratio
a + b- Total number of parts, which becomes the denominator
× 100- Converts each fractional share into a percentage
How to use this calculator
- Enter the first term of the ratio.
- Enter the second term.
- Read the first side's percentage of the whole on the gauge.
- Use the extras for the second share and for a as a percentage of b.
Worked examples
Splitting 3:2
The parts add to 5, so the first side is 3/5 = 0.60 of the whole, or 60%. The second side takes 40%. Read against each other rather than the whole, a is 150% of b.
A 7:9 workload split
Total parts come to 16, giving shares of 43.75% and 56.25%. The percentages add to 100%, which is the check worth doing every time — if your two shares do not sum to 100 you have divided by the wrong denominator.
How to read your score
Frequently asked questions
Why do I add the two terms together first?
Because a percentage is a share of the whole, and the whole is every part combined. In 3:2 there are five parts altogether, so each single part is worth 20% and the sides come to 60% and 40%.
When would I want a as a percentage of b instead?
When the two numbers are separate totals rather than slices of one pool. Comparing this year's output against last year's is that kind of question, and there the answer can exceed 100%.
Do the two percentages always add to 100?
For a two-part ratio, yes, up to rounding. If they do not, the sum used as the denominator was wrong. Rounding each to two decimals can leave a hundredth of a percent unaccounted for.
Can I use decimals in the ratio?
Yes. The share calculation works for any non-negative numbers. The simplified ratio shown in the extras only appears in reduced form when both terms are whole numbers.
Common ratios as percentage shares
| Ratio | Total parts | First share | Second share |
|---|---|---|---|
| 1:1 | 2 | 50.00% | 50.00% |
| 2:1 | 3 | 66.67% | 33.33% |
| 3:1 | 4 | 75.00% | 25.00% |
| 3:2 | 5 | 60.00% | 40.00% |
| 4:1 | 5 | 80.00% | 20.00% |
| 5:3 | 8 | 62.50% | 37.50% |
| 7:9 | 16 | 43.75% | 56.25% |
| 9:11 | 20 | 45.00% | 55.00% |
Each row's two shares add to 100% because both parts come from the same total.