Ratio Calculator
Solve, simplify, and scale ratios — find the missing value in A : B = C : D.
: = :
Leave exactly one box empty to solve for it.
Result
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About this tool
Two ratio tools in one. "Solve" finds the missing value in a proportion — type three of the four numbers in A : B = C : D and leave one blank. "Simplify" reduces a ratio to its lowest whole-number terms (and shows the decimal factor), handy for aspect ratios, mixing ratios, scale models, and recipe scaling. Calculations run entirely in your browser.
Frequently asked questions
- How does solving work?
- It cross-multiplies. For A : B = C : D, the missing term is found from A × D = B × C — so D = B × C ÷ A, and likewise for the others.
- Can I simplify a ratio with decimals?
- Yes. Decimal inputs are scaled up to whole numbers first, then reduced by their greatest common divisor, so 2.5 : 5 becomes 1 : 2.
- Why does my simplified ratio still have a decimal?
- If the two numbers have no common whole-number factor after scaling — for example an irrational-looking pair — the tool falls back to showing 1 : n with n rounded.