About this tool
Fill in three of the four terms of a/b = c/d and leave exactly one blank. It cross-multiplies to solve for the missing value and shows the answer live, entirely in your browser.
Frequently asked questions
How is the missing value found?
By cross-multiplication: a/b = c/d means a×d = b×c, so the blank term is found by rearranging that equation.
Why must exactly one field be blank?
A proportion has one unknown. If zero or more than one field is empty there is nothing unique to solve, so you are prompted to leave just one blank.