Proportion Calculator — Solve for X in Any Ratio
Solve the unknown value in any proportion (a:b = c:d) by cross-multiplication.
Tool Interface
Proportion Calculator
Leave one box empty — the calculator solves it by cross-multiplication.
How to use
- Fill in any three of the four boxes (a, b, c, d) and leave the unknown one empty.
- Hit Solve — the missing value is found by cross-multiplication.
- Example: to solve 3/4 = x/12, type 3, 4, leave the third box empty, type 12.
Everything runs in your browser — no data leaves your device.
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About This Tool
Introduction
Our free proportion calculator solves the unknown value in any proportion (a:b = c:d) instantly. Whether you are working on math homework, scaling a recipe up or down, or solving ratio problems at work, this tool finds the missing value by cross-multiplication and shows you the steps. It runs entirely in your browser — free, client-side, and with no signup required.
How to Use
- Fill in any three of the four boxes (a, b, c, d), leaving the unknown one empty.
- Click the Solve button, or press Enter.
- Read the missing value, displayed in large type along with the cross-multiplication steps.
- Example: to solve 3/4 = x/12, type 3, 4, leave the third box empty, and type 12.
- Click Reset to clear everything and start over.
Key Features
- Solve for any value: leave any one of the four boxes empty and the calculator solves it.
- Visual fraction layout: a clean a/b = c/d display that matches how you learned proportions in school.
- Cross-multiplication steps: shows the working (a × d = b × c) so you can follow along.
- Smart validation: friendly messages if you leave too many boxes empty or enter zeros in denominators.
- Mobile friendly: works beautifully on desktop and mobile, with dark-mode support built in.
Why Choose Trencada
Trencada’s calculators are free forever, require no account, and run entirely on your device — your numbers never leave your browser. The clean, distraction-free design and accurate math make this the tool students, teachers, and professionals come back to for quick, reliable answers.
FAQ
What is a proportion? A proportion states that two ratios are equal, written as a/b = c/d. When three of the four values are known, the fourth can always be found.
How does cross-multiplication work? Multiply the numerator of one side by the denominator of the other (a × d = b × c), then isolate the unknown value. The calculator does this automatically.
Can I use decimals? Yes — the tool accepts whole numbers and decimals, so recipe scaling and real-world ratios work fine.
Is my data stored anywhere? No. Everything is computed locally in your browser, so nothing is uploaded, tracked, or stored.