Cross-Multiply Solve Calculator
Solve a proportion a/b = c/x for the missing value by cross-multiplying — enter three numbers and get the fourth.
How it works
For a/b = c/x → x = c × b ÷ a (cross-multiply, then divide)
A proportion says two ratios are equal. Cross-multiplying replaces the equality of fractions a/b = c/x with the equality of products a·x = b·c, which is easy to solve for the unknown. It is the workhorse behind unit conversions, scaling, map distances and mixing problems — anywhere two quantities stay in a fixed ratio.
Worked example
To solve 3/4 = 9/x, cross-multiply to get 3·x = 4·9 = 36, then divide by 3 to find x = 12.
Frequently asked questions
Which position is the unknown?
This tool solves for x in the bottom-right of a/b = c/x. Arrange your problem so the unknown sits there, then enter the other three numbers.
Why must a not be zero?
The solution divides by a. If a were zero the left ratio a/b would be 0, forcing c to be 0 as well, and x could be anything — so the problem has no single answer.
Does cross-multiplication work with decimals?
Yes. Cross-multiplication is just algebra; it works for whole numbers, decimals and negatives alike.
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This calculator is for educational purposes. Double-check important results before acting on them.