Rule of Three Calculator

Direct and inverse proportion solved step by step.

Rule of three: simple and compound

Unknown value

ABCX = ?

Inverse (e.g. workers and days)

Direct: more A needs more B. Inverse: more A needs less B.

X equals12

Step by step

1. Direct relation: both grow together.

2 → 46 → 12

2. X = 4 × 6 ÷ 2 = 12

Solve simple and compound rules

Choose simple for one proportion (A → B, C → X) or compound for two quantities at once.
Pick the unknown value and tick inverse for workers-days or speed-time cases.
Read the step-by-step breakdown to check the setup and the equation.

Direct or inverse?

Direct when both grow together (liters and km). Inverse when one shrinking grows the other (workers and days).

When do I use the compound mode?

With two causes at once, e.g. 5 workers × 8 hours a day finish in 12 days: change workers and hours together to find the new deadline.

Can I solve for A, B or C instead of X?

Yes: pick the unknown in the dropdown and fill the other three values — the equation rearranges itself.