Proportion Calculator
Solve any proportion or ratio for the missing value, cross multiply, and check if two ratios are equal free in your browser with this proportion calculator.
Enter Values
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How to use this calculator
- Enter a, b and c.
- Press Calculate for x.
How it works
Solves a : b = c : x for the unknown: x = b × c ÷ a.
Worked example
2 : 4 = 3 : x. x = 6.
Common mistakes
- Cross-multiplying the wrong terms — keep matching quantities in the same positions.
- Assuming any two ratios are proportional; check they actually scale together.
Frequently asked questions
How do I solve a proportion?
Set the two ratios equal and cross-multiply: a/b = c/x gives x = (b × c) ÷ a.
What is a proportion?
A statement that two ratios are equal, such as 2/4 = 3/6 — useful for scaling recipes, maps and mixtures.
How does cross-multiplication solve this?
For a : b = c : x, multiply across: a × x = b × c, so x = b × c ÷ a. That's exactly how this finds the unknown.
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- Decimal to Fraction Converter
- Scale Converter
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Cite this tool
Zerdly. (2026). Proportion Calculator. Retrieved from https://www.zerdly.com/tools/proportion-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.