Solve a proportion of the form a/b = c/d when one of the four values is unknown.
Fraction arithmetic, decimal-to-fraction conversion, ratios and proportions.
Enter the values requested above and select Calculate. Change any input to compare another scenario. The calculation runs locally in your browser and does not require an account.
If 3/4 = x/20, then x = 15.
Solve a proportion of the form a/b = c/d when one of the four values is unknown. The result is intended for quick estimates, checking arithmetic and comparing scenarios. When a result affects a contract, tax filing, medical decision, building-code requirement or other high-stakes situation, verify the result with the appropriate professional or authoritative source.
If 3/4 = c/20, cross multiplication gives 4c = 3 × 20, so c = 15. Select c as the missing term; the calculator solves the equation while treating that input as unknown.
Check before trusting the result: the denominators of a valid proportion cannot be zero. If a relationship is not proportional (for example, it contains a fixed setup fee), cross multiplication alone does not model it correctly.
Solve a proportion of the form a/b = c/d when one of the four values is unknown.
Cross multiplication gives a×d = b×c. Rearrange that equation to solve for the missing value.
Yes. ERICH Calculators is free to use and does not require an account.
See how formulas, sources and limitations are handled across ERICH Calculators.