Formula
a ÷ b = c ÷ d · a × d = b × c
Set the two quotients equal. Cross multiplication gives a × d = b × c, from which the empty value can be isolated.
Ratio & proportion
● Runs locallyProportion Solver
Solve a rule of three by leaving exactly one position empty in a:b = c:d. CalcFlow isolates that position, shows the cross product and verifies the completed equality without evaluating free-form expressions.
Example values
Missing proportion value
Assumptions
- Exactly one value is empty.
- Zero cases that are indeterminate or inconsistent are rejected with a specific explanation.
Denominator positions b and d cannot be zero. Compatible units should be arranged consistently on both sides.
Formula review
Formula v1.0.0
Reviewed 2026-08-03
CalcFlow engineering review
Proportion Solver: Worked example
For 4:10 = x:25, cross multiplication gives 10x = 4 × 25 = 100, so x = 10.
Frequently asked questions
Which box should be empty?
Any one of a, b, c or d may be empty, but exactly one must be empty.
Why can a denominator not be zero?
A ratio written as division by zero is undefined.
Can I enter an equation?
No. The four approved numeric positions prevent arbitrary expression execution.