You can't solve — the proportion is undefined. The calculator shows "—".

"If 2 lbs of beef feeds 3 people, how many pounds for 12?" Set up 2/3 = x/12. Solve: x = (2·12)/3 = 8 lbs.

Does this work with units?

Yes — as long as the units on the left and right sides match. 2 km / 3 hr = x km / 12 hr.

How is this different from ratio-calculator?

ratio-calculator simplifies a:b. proportion-solver finds the missing fourth term in a:b :: c:x.

You can't solve — the proportion is undefined. The calculator shows "—".

"If 2 lbs of beef feeds 3 people, how many pounds for 12?" Set up 2/3 = x/12. Solve: x = (2·12)/3 = 8 lbs.

Does this work with units?

Yes — as long as the units on the left and right sides match. 2 km / 3 hr = x km / 12 hr.

How is this different from ratio-calculator?

ratio-calculator simplifies a:b. proportion-solver finds the missing fourth term in a:b :: c:x.

Proportion Solver

Given a/b = c/x (or any of three knowns), solve for the fourth.

Result

2/3 = 8/12.

Equationa/b = c/x
Cross-multiplya · x = b · c
Solvedx = (b · c) / a = (3 × 8) / 2

Step-by-step

Set up the proportion: 2/3 = 8/x.
Cross-multiply: 2 · x = 3 · 8 = 24.
Divide by a: x = 24 / 2 = 12.

How to use this calculator

Enter a, b, and c.
The calculator solves for x in a/b = c/x.

About this calculator

Two ratios are proportional if a/b = c/d. If three of the four values are known, cross-multiplication solves for the fourth: x = (b · c) / a. This is the foundation of "is the same as" reasoning — recipes scaled by ratio, map distances, mixing dilutions.