How to use this calculator
Leave one field blank and enter three positive values. The calculator identifies the unknown and solves the equation A×D = B×C. It also shows the completed proportion and the equal cross products for verification.
Formula
A : B = C : D ⇔ A × D = B × C
Worked example
For 2:3 = 8:D, D = 3×8÷2 = 12. Both cross products are 24, so 2:3 and 8:12 describe the same relationship. An answer such as 1/3 is shown as an exact fraction alongside its decimal approximation.
Assumptions and limitations
Exactly one field must be empty. The three supplied values must be positive; degenerate zero or negative proportions are outside this tool. Fractional answers are allowed. Convert quantities to consistent units before comparing ratios.
Frequently asked questions
Which field can be missing?
Any one of A, B, C or D. Leave it blank. Do not enter zero as a placeholder, because zero is a number rather than an unknown.
How precise are the results?
Finite decimal inputs are converted to fractions and reduced with integer arithmetic. Fractions and reduced ratios are exact within the input limits. Decimal displays use up to six places, or scientific notation for tiny nonzero results.
Does 2:3 mean the first part is two thirds of the total?
No. In a part-to-part ratio of 2:3, the total has five parts. The first share is 2/5 (40%) and the second is 3/5 (60%).
Exact reduction; decimal displays may be rounded.