Proportion Solver
Solve proportions of the form A/B = X/C for the unknown X. Instantly find the missing value in any proportion.
How to use this tool
- Enter A and B (the known fraction).
- Enter C (the denominator of the unknown fraction).
- Read X, the missing numerator.
Solve the proportion A/B = X/C to find the unknown X instantly.
Formula
Given the proportion A / B = X / C, solve for X:
X = (A ÷ B) × C
How it works
This calculator solves a simple proportion by computing the unit rate A / B and then multiplying by C to find the unknown fourth value X. It is equivalent to the classic cross-multiplication rule (A × C = B × X) rearranged for X. The tool assumes B is non-zero; entering zero for B will produce an undefined result.
Worked example
Worked example
- Proportion: 3 / 4 = X / 12.
- Unit rate = 3 ÷ 4 = 0.75.
- X = 0.75 × 12 = 9.
X = 9
Key terms
- Proportion
- A statement that two ratios are equal: A/B = X/C.
- Cross-multiplication
- A method for solving proportions: multiply diagonally (A × C = B × X) to isolate the unknown.
- Unit rate
- The value of A per single unit of B (i.e., A ÷ B), used as the scaling factor to find X.
- Missing value
- The unknown X in a proportion, determined from the three known quantities A, B, and C.
Frequently asked questions
- How do I solve a proportion?
- Cross-multiply: A/B = X/C means A × C = B × X, so X = (A × C) / B. Example: 3/4 = X/12 → X = (3 × 12) / 4 = 9.
- What is a proportion used for?
- Proportions are used in scaling recipes, map distances, currency conversions, mixing solutions, and calculating probabilities.