Proportions Calculator

Solve proportions by entering any three values and leaving one blank. The calculator will find the missing value.

Proportion Format: A : B = C : X
Enter any three values, leave one blank to solve for it

Worked example: solving for X

The proportion format is A : B = C : X. Enter A = 2, B = 3, C = 10, and leave X blank. Cross-multiplying A:B = C:X gives A × X = B × C, so:

X = (B × C) / A = (3 × 10) / 2 = 30 / 2 = 15

Check it: 2:3 = 10:15. Both ratios simplify to the same value (2/3 ≈ 0.667 and 10/15 ≈ 0.667), confirming the proportion holds.

The four solving formulas

Depending on which field you leave blank, the calculator uses a different rearrangement of the same cross-multiplication:

All four come from the same underlying identity A×X = B×C — the calculator just isolates a different variable depending on which box you leave empty.

Why exactly three fields, no more, no fewer

A proportion with two ratios has four numbers total; knowing any three uniquely determines the fourth. Filling all four leaves nothing to solve for, and filling only two or one leaves the equation under-determined — the tool enforces exactly three filled fields for this reason, not as an arbitrary restriction.

Division-by-zero cases

Solving for A divides by X, for B divides by C, for C divides by B, and for X divides by A — so whichever value sits in the denominator for your chosen unknown cannot itself be zero. The tool checks this and returns a specific error naming which field caused the problem.