grammatical number - Should we use plural or singular for a fraction of a mile?

I have seen people say both 0.25 mile and 0.25 miles. Should we use plural or singular for a fraction of a mile?

Answer

0.25 miles.

The rule I follow is that you use the singular only when talking of exactly one mile. You'd also say half a mile, quarter of a mile, etc., but in these cases it's as though you're still talking of "a mile" first, and then taking half or quarter of it.

Let me say something further, with the caveat that it may be my own idiosyncratic usage: use the singular only when talking of the natural number 1, that is, when you're essentially "counting" miles and there's a single one of them. Use the plural in all other cases, even for the real number 1.0: just like "0.9 miles" or "1.1 miles", also say "1.0 miles" (but "1 mile").

[Mathematically, natural numbers ("counting numbers") are a subset of real numbers, but more properly what we have is an inclusion map, so it does make sense to distinguish between 1 and 1.0, actually. And the distinction between natural numbers and real numbers does have applications elsewhere in English: consider "fewer than" versus "less than". You'd say "fewer than five items" but "less than 5.0 inches", since a length doesn't have to be an integer number of inches, you're not counting but assigning a real value.]

Another aside: the SI standard for symbols is to use the same symbol for singular or plural, so you'd write "2 km" even if you were reading it as "2 kilometres". Of course, since you're using miles, you probably don't care about this. :-)