How to convert a ratio to a percentage
A ratio written a:b is just a division waiting to happen. Turning it into a percentage takes two steps:
- Divide the first number by the second. For 3:4 that is 3 ÷ 4 = 0.75. This is the ratio as a decimal.
- Multiply by 100. 0.75 × 100 = 75%. Add the percent sign and you are done.
The whole operation is a single sum: (a ÷ b) × 100.
Part-to-part or part-to-whole? It changes the answer
This is where most mistakes happen, and the two readings give genuinely different numbers.
Say a team has 12 developers and 8 designers.
- Part-to-part (12:8) compares the two groups with each other. 12 ÷ 8 = 1.5, so 150% — there are half again as many developers as designers.
- Part-to-whole (12:20) compares developers with the whole team of 20. 12 ÷ 20 = 0.6, so 60% — developers make up 60% of the team.
Both are correct; they answer different questions. If your result comes out above 100% and that feels wrong, you probably wanted part-to-whole — switch to the Part of a total mode above.
Converting a percentage back to a ratio
Put the percentage over 100 and reduce the fraction. 75% becomes 75/100; both sides divide by 25, giving 3:4. 40% becomes 40/100, which reduces to 2:5. The calculator finds the greatest common divisor for you and scales decimals up to whole numbers where it can.
Common ratios and their percentages
| Ratio | Decimal | Percentage |
|---|---|---|
| 1:2 | 0.5 | 50% |
| 1:4 | 0.25 | 25% |
| 3:4 | 0.75 | 75% |
| 1:1 | 1 | 100% |
| 2:1 | 2 | 200% |
| 3:1 | 3 | 300% |
| 1:3 | 0.3333 | 33.333% |
| 5:4 | 1.25 | 125% |
| 1:5 | 0.2 | 20% |
| 2:5 | 0.4 | 40% |
| 7:10 | 0.7 | 70% |
| 9:10 | 0.9 | 90% |
Where this comes up
- Mixing and dilution — a 1:4 concentrate means one part product to four parts water, or 25% strength by that measure.
- Finance — debt-to-equity, current ratio and similar figures are often quoted both ways in the same report.
- Screens and images — a 16:9 aspect ratio is 177.8% wider than it is tall.
- Recipes and scaling — converting a ratio to a percentage makes it easy to scale a mix up or down.
- Odds and probability — odds of 1:3 mean one favourable outcome for three unfavourable, which is a 25% chance, not 33%.
Frequently asked questions
How do you turn a ratio into a percentage?
Divide the first number by the second, then multiply by 100 and add a percent sign. For 3:4, that is 3 ÷ 4 = 0.75, and 0.75 × 100 = 75%.
Why can a ratio give a percentage over 100?
Because a ratio compares two parts against each other, not a part against a whole. In 12:4 the first quantity is three times the second, so the answer is 300%. Only when the second number represents the total can the result be capped at 100%.
What is the difference between part-to-part and part-to-whole?
If a class has 12 boys and 8 girls, the part-to-part ratio is 12:8 — that is 150%, meaning there are half again as many boys as girls. The part-to-whole ratio is 12:20, which is 60% — the share of the class that is boys. Use the "part of a total" mode above for the second kind.
How do I convert a percentage back to a ratio?
Write the percentage over 100 and simplify. 75% becomes 75/100, which reduces to 3:4 by dividing both sides by 25. The reverse mode above does this and finds the simplest whole-number form.
Can I use decimals in a ratio?
Yes. 2.5:1 is perfectly valid and gives 250%. The simplified form shown will scale decimals up to whole numbers where it can — 2.5:1 is displayed as 5:2.
What happens if the second number is zero?
Nothing useful — dividing by zero is undefined, so no percentage exists. The calculator will tell you rather than showing a misleading result.