Proportion calculator
Solve a proportion of the form a/b = c/x. Find the missing fourth term from three nonzero numbers.
Result for the values shown.
Calculations run in your browser.
What to enter
| Input | Meaning and units |
|---|---|
| First numerator a | Enter first numerator a using the definition in the method and example. |
| First denominator b | Enter first denominator b using the definition in the method and example. |
| Second numerator c | Enter second numerator c using the definition in the method and example. |
Understanding your result
Read Missing denominator x in the units and model stated on this page. The formula x = b × c ÷ a defines the relationship; measurement accuracy and model applicability are separate from numerical precision.
Common mistakes
- Using a different mathematical definition or operation than the one stated by this tool.
Check your calculation
- Use the worked example as a known reference case, then change one input at a time and check the direction and units of the response.
Calculation checks, sources and review limits
What number makes these two ratios equal?
Enter a, b and c to solve for x in a/b = c/x. Keep corresponding terms in the same order and use consistent units.
Common uses
- Scale a ratio to a new quantity.
- Find a missing fourth proportional term.
How it works
Cross-multiply to get a × x = b × c, then divide by a. This form requires nonzero terms so both ratios are defined. Keep corresponding terms in the same order, and use consistent units across equivalent quantities.
Worked example
If 3/4 = 6/x, then x = 4 × 6 ÷ 3 = 8.