Proportion Calculator
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Result
Simplify: convert decimal terms to integers, divide both by their greatest common divisor. Solve: ? = B × C / A. Scale: k = target / A, target / B or target / (A + B); new ratio = (A × k):(B × k).
Common Core mathematics standard 6.RP.A.3 describes equivalent ratios. The formulas above follow by applying the same scale factor to both terms.
Worked example: Solve 2:3 = 8:?. The scale factor is 8 / 2 = 4, so the missing term is 3 × 4 = 12. For a recipe, 2 cups to 3 cups becomes 8 cups to 12 cups.
Terms must be nonnegative, with at least one nonzero. To solve C:?, A must be greater than zero. A scale target can set A, B, or their combined total. Use the same units before comparing terms; scaled amounts keep those units. Decimal simplification uses the entered number representation exactly; solved and scaled values use decimal arithmetic.
How to use this tool
Start in solve mode. Enter A, B and C in A:B = C:?, then Calculate the missing fourth term. Match units for corresponding terms.
Frequently asked questions
How is the missing term computed?
The missing fourth term equals B times C divided by A. This preserves the scale factor between the two equivalent ratios.
Can A be zero?
No. Solving the fourth term divides by A, so A must be greater than zero. B or C may be zero.
Can I simplify a ratio too?
Yes. The form also has simplify and scale modes. Solve mode is selected when this page opens.