If a/b = c/d, then the missing value is found by cross-multiplication (e.g. d = b×c/a when d is missing).

Which value can be unknown?

You can leave any one of a, b, c, or d blank; the tool finds it.

Can denominators be zero?

No. The known value in the denominator position must not be zero.

Solve for the missing term in two equal ratios (proportion): a:b = c:d. Enter three values to find the fourth.

a : b = c : d — leave one blank to solve.

How does it work?: If a/b = c/d, then the missing value is found by cross-multiplication (e.g. d = b×c/a when d is missing).
Which value can be unknown?: You can leave any one of a, b, c, or d blank; the tool finds it.
Can denominators be zero?: No. The known value in the denominator position must not be zero.

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