How this calculator works
Two fractions are equal precisely when their cross products agree, provided their denominators exist. Isolate the selected term in ad = bc. A zero divisor in the rearranged equation can make the solution absent or non-unique, so it produces an error instead of an arbitrary answer.
Formula
a/b = c/d gives ad = bc. Therefore a = bc/d, b = ad/c, c = ad/b, d = bc/a.
Worked example
For 2/5 = 6/d, the cross products give 2d = 30, so d = 15. In a/4 = 3/6, a = 4 × 3/6 = 2.
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Frequently asked questions
Can numerators be zero?
Yes when they still determine the chosen unknown uniquely. An all-zero numerator equation cannot determine a denominator.
Do the units matter?
Each ratio must compare corresponding quantities in consistent units before you set them equal.
Why is a zero solved denominator rejected?
Cross products alone can hide a division by zero. A valid proportion requires both original denominators to be defined.