Rule of Three Calculator
Solve direct or inverse proportion problems: if A is to B, what is C to X?
Inputs
Allowed range: -1000000000 to 1000000000
Allowed range: -1000000000 to 1000000000
Allowed range: -1000000000 to 1000000000
Results
X (unknown value)
15.0000
Formula used
X = (B × C) / A = (6 × 5) / 2
How it works
The rule of three solves a proportion with three known values and one unknown. Direct proportion: A/B = C/X ⇒ X = (B·C)/A. Inverse proportion: A·B = C·X ⇒ X = (A·B)/C.
Complete guide
Direct proportion: as one quantity grows, so does the other (e.g., 2 kg of flour costs $6, how much does 5 kg cost?).
Inverse proportion: as one grows, the other shrinks (e.g., 4 workers finish in 6 days, how long for 8 workers?).
Enter A, B and C, then pick the proportion type. We return X.
Frequently asked questions
- How do I know which type to pick?
- Ask: if A doubles, does C double too (direct) or halve (inverse)?
- Can it handle negatives?
- Yes, but proportion problems usually involve positive quantities — check that the result makes physical sense.
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