In case you didnt know already, the Google calculator will handle rack units. Just type your equation into the search engine in the form of a question. ex: "17 inches in RU" or "23RU in Millimeters". This will work for practically any form of measurement conversion BTW just in case you want to know how many "Stone" you weigh or how many "hands" tall your kids are.
For example, I just learned that I am 39.4285714 Rack units tall. That should impress the ladies.