Proportion Calculator
Proportion Calculator find missing proportion value a over b equals c over d ratio equation solverSolve one blank in two equal ratios
Enter three finite numbers in a ÷ b = c ÷ d and leave one box blank. Denominators in the completed proportion must not be zero.
How cross multiplication works
When a/b = c/d, multiplying both sides by b × d gives a × d = b × c. With exactly one unknown, that cross-product relationship can be rearranged to find the missing value. The calculator shows both completed ratios and their cross-products so the equality can be checked.
Worked example
Enter 3 for a, 4 for b, leave c blank, and enter 20 for d. The missing value is 15 because 3 × 20 equals 4 × 15; both cross-products are 60. The completed proportion is 3/4 = 15/20.
Zeros need care
A denominator cannot be zero because division by zero is undefined. Some zero-numerator cases also fail to identify one unique missing denominator. For example, 0/b = 0/5 is true for many nonzero values of b, so one unique b cannot be solved. The page rejects cases where the rearranged calculation divides by zero or does not produce a finite unique value.
Ratios can carry units
The fields accept numbers only, so keep unit meaning outside the form. Compare quantities in compatible arrangements: miles over hours should correspond to miles over hours, not hours over miles. A correct arithmetic proportion can still be meaningless when units or categories do not match.
Rounding and precision
Results use JavaScript decimal-number arithmetic and display up to twelve significant digits. Repeating decimals and very large or small magnitudes may be rounded for display. Substitute the displayed value back into the original relationship when precision matters, and retain exact fractions manually when exact symbolic form is required.
Limitations and privacy
All values remain in this browser. The tool solves one numeric unknown only. It does not parse equations, preserve fractions symbolically, attach units, propagate measurement uncertainty, find multiple solutions, or replace subject-specific reasoning about whether a proportional relationship is valid.
Proportion Calculator FAQ
Can I leave two boxes blank?
No. Three known values are needed to find one unique value.
Can values be negative?
Yes, when denominators remain nonzero and the result is finite.
Why was a zero case rejected?
It either created division by zero or did not determine one unique missing value.