Cross Multiplication Calculator — Solve Proportions a/b = c/d
Solve any proportion a/b = c/d for the missing value using cross multiplication. Enter any 3 known values and find the unknown with step-by-step solution.
Enter any 3 values and leave the unknown blank.
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Verification
Cross Multiplication Steps
Frequently Asked Questions
What is cross multiplication?
Cross multiplication is a method to solve proportions of the form a/b = c/d. You multiply the numerator of one fraction by the denominator of the other: a×d = b×c. This gives a simple equation to solve for the unknown.
How do I solve a/b = c/d for an unknown?
Cross multiply to get ad = bc. Then isolate the unknown. For example, if d is unknown: d = bc/a. If a is unknown: a = bc/d. This calculator handles all four cases.
When is cross multiplication used?
Cross multiplication is used for: solving proportions, scaling recipes, currency conversion, map scale problems, unit conversion, and comparing fractions (a/b vs c/d — cross multiply to compare).
Can I use cross multiplication to compare fractions?
Yes. To compare a/b and c/d: compute a×d and b×c. If a×d > b×c, then a/b > c/d. If a×d < b×c, then a/b < c/d. If equal, the fractions are equivalent.
What is the difference between proportion and cross multiplication?
A proportion is the statement that two ratios are equal: a/b = c/d. Cross multiplication is the technique used to solve it — by multiplying across the equality to eliminate fractions.