➗ Proportions 🧮 Math Updated2026-07-18
Ratio Calculator.
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Quick answer
A proportion A : B = C : D means A/B = C/D. To find a missing value, cross-multiply: A×D = B×C. Enter any three values below and leave one blank to solve it. The tool also simplifies A : B to lowest terms.
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Ratio Calculator
Solve the proportion: leave one box blank
:=:
Solved for D
D = 8
Ratio A : B simplified → 1 : 2
✨ Live · Enter any three of A : B = C : D and leave the fourth blank. Also reduces A : B to lowest terms.
📊 Worked example
Scaling a 2 : 3 ratio.
Multiply both parts by the same number and the ratio stays equivalent, the basis for doubling recipes or mixing paint.
| Scale | Part A | Part B |
|---|---|---|
| ×1 | 2 | 3 |
| ×2 | 4 | 6 |
| ×3 | 6 | 9 |
| ×4 | 8 | 12 |
| ×5 | 10 | 15 |
| ×10 | 20 | 30 |
❓ FAQ
Common questions.
How do you solve a ratio proportion?
A proportion says two ratios are equal: A:B = C:D, which is the same as A/B = C/D. Cross-multiply to get A×D = B×C, then solve for the unknown. For example, if 2:3 = 8:x, then 2×x = 3×8 = 24, so x = 12. Enter any three of the four values above and leave the fourth blank. The calculator cross-multiplies for you.
How do you simplify a ratio?
Divide both sides by their greatest common divisor (GCD). For 12:18, the GCD is 6, so it simplifies to 2:3. A ratio is in simplest form when the only whole number dividing both parts is 1. For ratios with decimals, first multiply both sides by 10, 100, etc. to clear the decimals, then reduce.
How do you scale a ratio up or down?
Multiply (or divide) both parts by the same number. To scale a 2:3 paint mix to make more, multiply both by 5 → 10:15, which keeps the same proportion. This is exactly how recipes are doubled or halved. The scaling table on this page shows the 2:3 ratio scaled from 1× to 10×.
What is the difference between a ratio and a fraction?
They're closely related but describe different things. A ratio A:B compares two quantities (e.g. 2 parts water to 3 parts cordial). A fraction expresses one part of a whole. That same mix is 2/5 water and 3/5 cordial. You can always turn a two-part ratio into fractions by putting each part over the total (2+3 = 5).
How do you convert a ratio to a percentage?
Add the parts to get the total, then divide each part by the total and multiply by 100. For 2:3, the total is 5, so the split is 40% (2/5) and 60% (3/5). This is useful for turning odds, mixes, and allocations into percentages.