Enter values and select Calculate.
Calculated result
Calculation steps
Formula and worked example
For a/b = c/d, equivalent ratios have equal cross products: a × d = b × c. The calculator isolates whichever one of a, b, c, or d is blank.
For 2/3 = x/12, cross multiplication gives 2 × 12 = 3x. Dividing 24 by 3 gives x = 8.
Calculations use full internal precision and trim insignificant trailing zeros for display. Confirm any rounding rule required by a course, workplace, publication, or technical standard.
How to use it
Enter three known real numbers in the a, b, c, and d positions.
Leave exactly one position blank to mark the unknown.
Select Calculate and review the completed proportion and cross-product steps.
Keep denominators nonzero and make sure compared quantities use compatible units.
Zero and undefined cases
A zero numerator is valid when the proportion remains defined.
A known denominator of zero is rejected because division by zero is undefined.
If the only solution would make an unknown denominator zero, the calculator reports that the proportion is undefined.
Some zero combinations do not determine one unique value; those are rejected rather than guessed.
Proportion versus ratio
A ratio compares quantities and can be simplified. A proportion states that two ratios are equivalent and often asks for a missing value.
Use the related Ratio Calculator when the primary job is simplifying a ratio rather than solving a proportion.
Methodology and limits
This deterministic calculator uses the formula shown above and strict finite-number parsing. It does not evaluate expressions, execute entered text, call a remote calculation service, or claim to replace instruction or professional review.