How to use this proportion calculator
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.
Formula and method
Example
If 3/4 = x/20, then x = 15.
Tips for a useful result
Set the “Solve for” field to the variable you want; the current value typed in that field is ignored.
What this calculator is useful for
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.
Frequently asked questions
What does the Proportion Calculator calculate?
Solve a proportion of the form a/b = c/d when one of the four values is unknown.
How does the Proportion Calculator work?
Cross multiplication gives a×d = b×c. Rearrange that equation to solve for the missing value.
Is the Proportion Calculator free to use?
Yes. ERICH Calculators is free to use and does not require an account.