Cross multiplication
In A : B = C : X, the two ratios are equal, so A times X equals B times C.
Divide both sides by A to get X = B x C / A.
Worked example
Solve 3 : 4 = 15 : ?. The cross product is 4 x 15 = 60, and 60 / 3 = 20, so the missing term is 20.
The scale factor is 15 / 3 = 5, which is why the second term also multiplies by 5 to reach 20.
Checking your answer
Both sides should reduce to the same ratio. 3 : 4 is already in lowest terms, and 15 : 20 reduces to 3 : 4.
The cross products must match too: 3 x 20 = 60 and 4 x 15 = 60.
Inputs for the proportion calculator
First term (A)
The first number of the known ratio. It cannot be zero, because the answer is divided by it.
Second term (B)
The second number of the known ratio, paired with A.
Third term (C)
The matching first number of the second ratio. The missing term is the one paired with it.
CALCULATOR QUESTIONS
Proportion calculator questions
How do I solve a proportion?
Multiply the second term by the third and divide by the first. For 3 : 4 = 15 : ?, that is 4 x 15 / 3 = 20.
What is a proportion?
Two ratios that describe the same relationship, such as 3 : 4 and 15 : 20.
Why can't the first term be zero?
The missing term is found by dividing by the first term, and division by zero has no answer.
Does this work for inverse proportion?
No. This calculator solves direct proportion, where both sides grow together. In inverse proportion one side falls as the other rises.