Cross Multiplication Calculator
Solve proportions (a/b = c/d) by cross-multiplying — enter any three values and get the fourth with step-by-step working.
Solve a proportion
Updates as you typeMode
Proportion type ?
a/b = c/d
Quick examples
Proportion
Enter any three values — leave one empty (or type
x) to solve ?
Display (optional)
Labels for each value ?
Decimal places
Formula
a
b
=
c
d
⇒
a
×
d
=
b
×
c
- a, b
- First pair of values (known ratio or product)
- c, d
- Second pair, one of which is the unknown
- Direct
- As one value grows, the other grows in the same proportion
- Inverse
- As one value grows, the other shrinks — their product stays constant
Worked example — your numbers
- Set up: —
- Cross-multiply: —
- Isolate unknown: —
- Compute: —
- Answer: —
Cross-multiplication works because multiplying both sides of an equation by the same quantity preserves equality. For a/b = c/d, multiplying both sides by b × d clears the denominators and leaves a × d = b × c — the diagonal products must match.