What is cross multiplication?
It is a method for solving proportions by equating the products of opposite terms.
Cross Multiplication Calculator
Solve for the missing value in a proportion using classic cross-multiplication.
Set up the proportion
Choose which term is missing, then enter the three known values.
About this calculator
How to use Cross Multiplication Calculator, plus the formulas and logic behind the results.
Tool description
Cross multiplication turns a proportion into an equation. For a/b = c/x, x = (b×c)/a. For a/b = x/d, x = (a×d)/b.
How to use
- Choose whether x is on the right or left of the proportion.
- Enter the three known values.
- Select Calculate to solve for x.
- Review the fraction and decimal forms of x.
Formulas & reference
a/b = c/x ⇒ x = (b × c) / aa/b = x/d ⇒ x = (a × d) / bCheck: a × x = b × c (or a × d = b × x)
Proportion tips
- Cross products must be equal
- 2/3 = 4/x ⇒ x = 6
- 2/3 = x/6 ⇒ x = 4
- Simplify the solved x when possible
Calculation logic
The selected proportion formula is applied with integer arithmetic, then simplified. Division by zero in the known terms is blocked.
Only one unknown is solved at a time. Unused fields are ignored based on the selected mode.
Cross Multiplication Calculator FAQ
Common questions about this calculator and how to use the results.
When do I use a/b = c/x?
Use that form when the unknown is the denominator on the right side.
When do I use a/b = x/d?
Use that form when the unknown is the numerator on the right side.
How do I check my answer?
Substitute x back into the proportion and confirm both sides match.
Can values be negative?
Yes. Signs are preserved through the algebra.
What if a known term is zero?
Some zero cases make the proportion undefined or force x to zero; invalid divisions are rejected.