Find the missing number
Solve for x in a proportion.
Have a proportion with a missing value, like x/4 = 3/6? Put x in the empty box and see it solved by cross-multiplication, step by step.
Solve the proportion
Put x in the box you want to find; fill the other three with numbers.
The steps
A proportion says two fractions are equal, such as x/4 = 3/6. To solve for the missing value, you cross-multiply: the diagonal products of a proportion are always equal. So for x/4 = 3/6, you get x × 6 = 4 × 3, which gives x = 2.
This works no matter which of the four positions is unknown — just type x in that box. If the answer isn't a whole number, it's shown as a fraction (and a decimal).
Worked examples
Frequently asked
How do you solve for x in a proportion?
Cross-multiply: multiply the numbers on each diagonal and set them equal. For x/4 = 3/6, that is x × 6 = 4 × 3, so 6x = 12 and x = 2.
What is cross-multiplication?
In a proportion a/b = c/d, the diagonal products a × d and b × c are always equal. Setting them equal lets you solve for any single missing value.
What if x isn't a whole number?
The calculator shows the exact answer as a fraction, plus an approximate decimal. For example, 2/3 = 3/x gives x = 9/2, which is 4.5.
Where is this used?
Proportions appear in equivalent fractions, scaling recipes, unit conversions, map scales and similar-shape geometry — anywhere two ratios must stay equal.