Math calculator

Ratio and Proportion Calculator

Enter A:B and the matching value C to solve A:B = C:X in one step.

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No account. Formula and assumptions are shown below.

Calculator inputs

Ratio details

Formula & disclaimer

For A:B = C:X, Missing Value X = C × B ÷ A

View assumptions

Formula reviewCalcs at Hand Editorial TeamVerified 28 August 2026 · automated checks included

Three useful checks, kept deliberately short.

What this result includes

The missing value is 15 and the equivalent ratio is 10:15.

A practical example

Solve 2:3 = 10:X: Find the scale factor 10 ÷ 2 · Multiply the second ratio value by that factor · Simplify the original ratio.

Common input mistake

The simplified ratio keeps the same relationship using the smallest supported whole-number pair.

For A:B = C:X, Missing Value X = C × B ÷ A

Equivalent ratios multiply or divide both sides by the same scale factor.

Keep in mind: All entered ratio values must be positive and use consistent units.

Produced byCalcs at Hand Editorial Team

Last formula check28 August 2026

Review methodKnown examples + boundary cases