Proportion Calculator

Solve A : B = C : D — enter any three values and find the missing term with cross-multiplication.

Enter any three values in A : B = C : D. The missing value updates as you type (cross-multiplication).

Proportion

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Your Results

Enter three numbers; leave one box empty to solve it.

What is a proportion?

A proportion says two ratios are equal. If A : B = C : D, then the cross-products match: A × D = B × C. You can leave any one value blank and calculate it from the other three—ideal for scaling recipes, maps, or any “same rate” problem.

FAQ

When is a proportion not valid?

If you enter all four numbers, we check whether A×D equals B×C. If not, the set is not proportional.

Can I use decimals?

Yes. The tool uses standard floating-point math; for money or scientific work, round only at the end.

Why did it say “cannot divide by zero”?

Solving for one letter sometimes divides by another part of the ratio. If that part is zero, the problem is undefined.

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