Proportion Solver
Solve a proportion for the unknown value using cross multiplication. Enter three numbers of a : b = c : x and get x instantly.
Last reviewed by the Radiatus Cloud team
Solve the proportion a : b = c : x for the missing value x using cross multiplication.
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Solve any proportion for the unknown
A proportion states that two ratios are equal, written as a : b = c : x or as the fractions a over b equals c over x. The Proportion Solver finds the missing fourth value using cross multiplication. From a over b equals c over x, cross multiplying gives a times x equals b times c, so x equals b times c divided by a. Enter the three known values and the tool returns x exactly.
This is the mathematical basis of the "rule of three", a method for scaling quantities up or down while keeping their relationship constant.
Everyday uses of proportions
Proportions appear everywhere. Cooks scale a recipe from four servings to six. Builders convert measurements on a scale drawing. Shoppers compare unit prices. Students solve similar-triangle and map-scale problems. Chemists dilute solutions to a target concentration. Any time two quantities change together at a fixed rate, a proportion finds the missing number.
The solver shows the rearranged formula and a check line so you can see that the two ratios really are equal. All calculation happens locally in your browser, with no rounding beyond standard floating-point precision.
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Frequently Asked Questions
What is cross multiplication?
It is the step that turns a over b equals c over x into a times x equals b times c, letting you solve for the unknown by simple division.
Which value does it solve for?
It solves for x, the fourth term in a : b = c : x. Rearrange your numbers so the unknown is in that position.
Why can a not be zero?
Solving for x requires dividing by a. Division by zero is undefined, so a must be a non-zero number.
Can I use decimals and negatives?
Yes. Any real numbers are accepted, so you can solve proportions involving decimals or negative values.
Is this the same as the rule of three?
Yes. The rule of three is the everyday name for solving a proportion for its missing fourth value, which is exactly what this tool does.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter a, b and c in the proportion a : b = c : x and the tool solves for x.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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