Proportion Calculator

A proportion states that two ratios are equal, written a/b = c/d. When three of the four values are known, the fourth is fixed and can always be found, which is exactly what this calculator does.

Whichever single box you leave blank. Leave a empty and it solves for a, leave d empty and it solves for d. You must leave exactly one box blank; leaving two or filling all four will not produce an answer.

From a/b = c/d, multiplying both sides gives a times d equals b times c. The calculator rearranges that identity: a = b*c/d, b = a*d/c, c = a*d/b, and d = b*c/a.

Yes. Any of the boxes accept decimals or negatives, and the result is rounded to six decimal places, so 5 / b = 8 / 12 returns b = 7.5 and other fractions display cleanly.

Solving for the blank means dividing by one of the other numbers. If that divisor is zero, for example solving for a when d is 0, no finite answer exists and the tool reports the error instead of a wrong number.

The tool shows Enter valid numbers until every filled box holds a real value. Blank means unknown, so an empty box is fine, but letters or stray symbols are flagged rather than guessed.

It solves direct proportions, where the two ratios stay equal, a/b = c/d. For inverse proportion, where one quantity rises as the other falls so a times b equals c times d, rearrange your numbers before entering them.

Scaling a recipe up or down, converting between units, reading distances off a map scale, adjusting mixing ratios, and working out similar-triangle side lengths are all direct-proportion problems this handles.

Yes. The calculation runs entirely in your browser. Nothing you type is uploaded, logged or stored.