Solve any proportion a/b = c/d — find the missing term instantly
Solve a / b = c / d
Click any field label (A / B / C / D) to set it as the unknown to solve for.
Quick examples
Result
How cross-multiplication works
If a / b = c / d, then a × d = b × c. Rearranging:
A proportion calculator solves the equation a/b = c/d when three of the four values are known. Whether you're scaling a recipe, converting units, solving a geometry problem, or working through a math class assignment, this free tool finds the missing term instantly using cross-multiplication and shows the full verification check.
A proportion is an equation stating two ratios are equal: a/b = c/d. To solve for a missing term, use cross-multiplication: multiply across the equals sign (a × d = b × c), then divide both sides to isolate the unknown. For example, to find d in 3/4 = 9/d: d = (4 × 9) / 3 = 12.
You can solve for any one of the four values — A, B, C, or D. Click the label of the value you want to find, enter the other three, and the answer appears instantly.
Yes. All four fields accept any positive or negative decimal number. The calculator handles values like 2.5, 0.333, or 1000 without rounding errors.
It verifies the result is correct by confirming that a × d equals b × c. If the two sides match, the proportion is exact. This is the standard algebraic method for verifying proportion solutions.