Ratio Calculator
Simplify ratios and solve proportions for missing values.
Quick answer
A ratio compares two quantities, like 3:4. Scale it by multiplying both sides by the same number; simplify by dividing both by their greatest common factor. Two ratios are equal (a proportion) when their cross-products match: a:b = c:d means a×d = b×c.
Simplified ratio
3 : 4
Solve: 3 : 4 = 9 : D
D = 12
As a fraction of the whole
42.9% / 57.1%
Scaling recipes and maps
Equivalent ratios come from multiplying both sides by the same factor — 3:4 = 9:12. Solving a proportion cross-multiplies: if 3:4 = 9:D then 3D = 36, D = 12. Same arithmetic scales recipes, map distances, and paint mixes.
Ratios vs. fractions
A 3:4 ratio means parts of 3 and 4 — so the first part is 3/7 of the whole, not 3/4 of it. Mixing those two readings is the most common ratio mistake; the part-of-whole percentages shown above keep them straight.
Frequently asked questions
- How does the Ratio Calculator work?
- A ratio compares two quantities, like 3:4. Scale it by multiplying both sides by the same number; simplify by dividing both by their greatest common factor. Two ratios are equal (a proportion) when their cross-products match: a:b = c:d means a×d = b×c.
- Is the Ratio Calculator free to use?
- Yes. The Ratio Calculator is completely free, runs entirely in your browser, and requires no account or sign-up. Your inputs never leave your device.
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