Rule of Three Calculator

It's a way to find an unknown value in a proportion. If a/b = c/d and one of them is unknown, you cross-multiply: a·d = b·c, then divide to isolate the missing value.

Anywhere — leave a, b, c or d empty. The calculator detects which field is blank and solves for it as long as that doesn't require dividing by zero.

Yes. Any of the values can be a decimal (e.g. 2.5), and the result keeps full precision, trimmed to a readable number of digits.

Solving for one position divides by another value. If that value is zero, the proportion has no finite solution — change the inputs so you're not dividing by zero.

Swap the two values in one of the ratios. The rule of three assumes direct proportion — double one side, double the other — but a lot of real problems are inverse: 4 workers take 6 days, so 8 workers take 3, not 12. Enter the workers in the opposite order to the days: a = 8, b = 4, c = 6, blank d, and you get 3. The test for which you have is whether the product a·b stays constant (inverse) or the ratio a/b does (direct).

Because any other count has no single answer. With two blanks the proportion has infinitely many solutions — 1/2 = x/y holds for every pair where y is twice x — so there is nothing to compute. With none there is nothing to solve for, though you can still read the equation as a claim to check by eye. Zero is refused only in the position that would become the divisor: solving for d divides by a, for c divides by b, for b divides by c and for a divides by d.