This proportion calculator solves a missing value in a ratio equation a:b = c:d using cross-multiplication.
Math
Proportion Calculator
Solve for the fourth term in a proportion a:b = c:d.
Calculator
About this calculator
Formula notes
a/b = c/d means a·d = b·c, so the unknown equals the product of the two known diagonal terms divided by the remaining term.
Worked examples
- Example: 2:3 = 8:d gives 2d = 24, so d = 12 — matching the page default.
- For 7/12 = x/36, cross-multiplication gives 12x = 252, so x = 21.
How to use it
- Fill any three of the four ratio terms.
- Leave the unknown term blank or solve for it.
- Read the completed proportion equation.
Frequently asked questions
What if more than one term is empty?
The equation is underdetermined; provide three values so one unique answer exists.
Can the terms be decimals?
Yes. Proportions work with any real numbers, not just whole numbers.
Limits and interpretation
- The relationship must genuinely be proportional; fixed fees, offsets, thresholds, and nonlinear effects violate the model.
- Terms that appear in denominator positions cannot be zero, and units on corresponding terms must match.