Quick answer
Add both parts (a + b = total), then divide each by the total and multiply by 100. For 3:5: total = 8; A = 3/8 × 100 = 37.5%; B = 5/8 × 100 = 62.5%.
How to use this calculator
Enter the two parts of the ratio (a and b). The calculator returns the percentage each part represents of the whole (a + b), along with decimal equivalents and the full step-by-step workings.
What is a ratio-to-percentage conversion?
A ratio describes how two quantities relate to each other in parts (e.g. 3:5 — three parts to five parts). A percentage expresses each part's share of the total as a value out of 100. Converting a ratio to percentages is the process of determining what fraction of the whole each part represents.
The two percentages always sum to exactly 100%. The conversion treats the ratio as a part-to-part relationship and implicitly calculates the whole (a + b).
Formula and method
For a ratio a:b, the percentage of each part is:
$$\%A = \frac{a}{a + b} \times 100$$
$$\%B = \frac{b}{a + b} \times 100$$
Example with 3:5:
$$3{:}5 \implies \frac{3}{8} \times 100 = 37.5\%\quad\text{and}\quad\frac{5}{8} \times 100 = 62.5\%$$
Example with 1:3:
$$1{:}3 \implies \frac{1}{4} \times 100 = 25\%\quad\text{and}\quad\frac{3}{4} \times 100 = 75\%$$
Worked examples
| Ratio | Total parts | Part A % | Part B % |
|---|---|---|---|
| 1:1 | 2 | 50% | 50% |
| 1:3 | 4 | 25% | 75% |
| 1:4 | 5 | 20% | 80% |
| 2:3 | 5 | 40% | 60% |
| 3:5 | 8 | 37.5% | 62.5% |
| 2:1 | 3 | 66.67% | 33.33% |
| 3:1 | 4 | 75% | 25% |
| 4:1 | 5 | 80% | 20% |
| 9:1 | 10 | 90% | 10% |
| 19:1 | 20 | 95% | 5% |
Common ratio reference table
| Ratio | Part A % | Part B % | Typical context |
|---|---|---|---|
| 1:1 | 50% | 50% | Equal split, 50/50 partnership |
| 2:1 | 66.67% | 33.33% | Two-thirds majority |
| 3:1 | 75% | 25% | Three-quarters allocation |
| 3:2 | 60% | 40% | Macronutrient split, investment allocation |
| 4:1 | 80% | 20% | Pareto principle (80/20 rule) |
| 2:3 | 40% | 60% | Debt:equity ratio |
| 1:9 | 10% | 90% | Tax rate (10% VAT) |
When to use this conversion
Finance and investment — a 60:40 portfolio (stocks:bonds) means 60% stocks and 40% bonds. Converting the ratio to percentages makes it clear what proportion of capital is allocated to each asset class.
Mixing and recipes — a cement:sand ratio of 1:3 means cement is 25% of the mix by volume. Understanding percentages is often more practical than ratios when measuring total quantities.
Statistics and surveys — survey results are often reported as ratios (3 in 5 support X). Converting to a percentage (60%) communicates the result more intuitively.
Nutrition — macronutrient ratios (protein:carbs:fat) are often converted to percentage shares for dietary planning (e.g. 30% protein, 40% carbs, 30% fat).
Frequently asked questions
How do you convert a ratio to a percentage?
Add both parts to get the total. Divide each part by the total and multiply by 100. For 3:5: total = 8; A = 37.5%, B = 62.5%.
What percentage is a 1:3 ratio?
Total = 4. A = 1/4 × 100 = 25%, B = 3/4 × 100 = 75%.
What percentage is a 2:1 ratio?
Total = 3. A = 2/3 × 100 ≈ 66.67%, B = 1/3 × 100 ≈ 33.33%.
Do the two percentages always add to 100%?
Yes — by definition, the two parts together form the whole (100%).
Can I use this for 3-part ratios?
This calculator handles 2-part ratios. For three-part splits (a:b:c) use the 3-part ratio calculator.