How this calculation works
Use cross multiplication for exact comparisons of integer fractions, even when both decimal approximations would round to the same number.
THE FORMULA
a/b = c/d exactly when ad = bcA worked example
3 × 12 and 4 × 9 both equal 36.
Good to know
Use positive denominators.
Reference: OpenStax Prealgebra (opens in a new tab) ↗
Read the result
Check a proportion by comparing the two diagonal products without rounding either ratio.
The primary result is labeled “Equal ratios?”. Keep the input definitions in view when comparing this result with another method.
Try a related calculation
Use positive denominators.
For a related exercise, try the fraction calculator.
A few questions, answered
How can I check the default inputs by hand?
3 × 12 and 4 × 9 both equal 36.
Which rule does this calculator apply?
a/b = c/d exactly when ad = bc