Here's another example: [68 f =]. The query is shorter than [68 f to c] or [6 fahrenheit to celsius]. Unfortunately, [20 c =] doesn't convert temperature, since c is the speed of light, so you'll have to use [20 c to f].
You can use Google's dropdown to pick other measurement units or you can edit the value and the result will change as yo type.
This trick also works in Chrome's omnibox, Firefox's search box, Safari's address bar: