Math calculator
Proportion Calculator
Solve a four-value proportion and see the cross multiplication used.
For a ÷ b = c ÷ d, leave exactly one box blank. b and d cannot be zero.
d equals
12
Cross multiplication2 × x = 3 × 8
How it works
Leave exactly one of a, b, c, and d blank. The calculator cross multiplies and isolates the missing value. Denominators cannot be zero; decimal and negative values are valid when the divisions remain defined.
a ÷ b = c ÷ d ⇒ a × d = b × c
Worked example
For 2/3 = 8/x, cross multiply: 2x = 24, so x = 12.
How to use this tool
- Enter the known values and choose units where available.
- Check the primary result and supporting details.
- Clear or replace an input to calculate another case.
Frequently asked questions
What happens with invalid or incomplete inputs?
The result remains blank until the required inputs form a finite, meaningful calculation. Zero denominators and physically impossible negative quantities are not calculated.
Can I use decimal values?
Yes. The tool accepts decimals and keeps full precision for the calculation while rounding the displayed result for readability.