Instantly solve the simple rule of three — direct or inverse proportion. Enter the known values A, B and C and find the unknown X.
Enter the known values
Direct: if A corresponds to B, then C corresponds to X. E.g. 2 kg → 10, so 5 kg → X.
X = 10 × 5 ÷ 2 = 25
2 → 10, 5 → 25Standard mathematical formulas. Instant in-browser calculation, no account, no data sent. Results are exact for the values you enter.
The rule of three solves a proportion: if A corresponds to B, then C corresponds to X. In direct proportion, X is found by cross-multiplication:
If 2 kg costs 10, how much is 5 kg? X = (10 × 5) ÷ 2 = 25. Inverse: 2 workers finish in 10 days, so 5 workers in (2 × 10) ÷ 5 = 4 days.
A method to find an unknown fourth value in a proportion when you know the other three: if A corresponds to B, then C corresponds to X.
By cross-multiplication: X = (B × C) ÷ A. If 2 kg costs 10, then 5 kg costs (10 × 5) ÷ 2 = 25.
When the quantities are inversely proportional (one grows, the other shrinks). X = (A × B) ÷ C. E.g. more workers finish in fewer days.
In direct proportion both quantities grow together; in inverse proportion, as one grows the other shrinks proportionally.
Yes. “What is 15% of 200?” becomes: 100 → 200, 15 → X, so X = (200 × 15) ÷ 100 = 30.
When more than two quantities are involved (e.g. workers, days and hours). This calculator solves the simple rule of three; for the compound one, apply the proportions one at a time.
Scale the ingredients proportionally. If a recipe for 4 servings needs 300 g of flour, for 6 servings you need (300 × 6) ÷ 4 = 450 g.
At a 1:100,000 scale, 1 cm on the map equals 1 km in reality. So 3.5 cm on the map is 3.5 km on the ground, using direct proportion.
In direct proportion the ratios must be equal: A ÷ B = C ÷ X. If that equality holds, the value of X is correct.