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. :-)
english.stackexchange.com/users/77/ShreevatsaR
Depends how you say it.
You'd say "nought point two five miles" (plural) but "a quarter of a mile" (singular)
english.stackexchange.com/users/866/Simon Whitaker
I'd leave it singular unless talking about a range such as "0.25 - 2 miles".
In a table, for example, the heading would be the same regardless of the units.
You can always abbreviate as "mi" and avoid the issue.
Simon's answer is also good.
Also note that it's actually more awkward to say 0.25 mile than "point two five of a mile".
english.stackexchange.com/users/161/Goodbye Stack Exchange
For reasons I'm not too sure about, we usually say "0.25 miles" rather than "0.25 mile".
If you Google for both terms, there are about 700,000 "0.25 miles" but less than 100,000 "0.25 mile", and the first few hits for "0.25 mile" give things like "0.25 mile swim" or "0.25 mile wireless setup", where the "mile" is forced to singular anyway.
english.stackexchange.com/users/None/
If we followed the definition of "plural", the correct usage would be: 0.25 mile 0.5 mile 0.9 mile 1.0 mile 1.3 mile (or 1.3 miles, according to Wiktionary note below) 2.0 miles 2.2 miles..
As the definition is: Plural: of, relating to, or constituting a class of grammatical forms usually used to denote more than one or in some languages more than two http://www.merriam-webster.com/dictionary/plural
However, as Shinto pointed out in his answer here, people usually use the plural form for 0.25.
Here is an interesting note from the Wiktionary about the usage of plural for fractions (and decimals) in general: While the plural form generally refers to two or more persons or things, that is not always the case.
The plural form is often used for zero persons or things, for fractional things in a quantity greater than one , and for people or things when the quantity is unknown.
This does support the usage of plural for 0.25 though, since it is less than one.
But if that is how people use it, we definitely can't say that it is incorrect.
english.stackexchange.com/users/212/b.roth
.25 miles or a quarter of a mile.
english.stackexchange.com/users/914/Picturepocket
It's really not complicated or confusing if you think of it in terms of being a fraction.
Essentially, as soon as you go into decimal, you're changing units into partial units.
0.25 would actually be 25 units of "hundredth-miles"
english.stackexchange.com/users/134/OneProton
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?
Should we use plural or singular for a fraction of a mile?
english.stackexchange.com
<grammatical-number><numbers><measuring-units><fractions>
english.stackexchange.com/questions/2139
english.stackexchange.com/users/165/grokus
The generally agreed rule is that you should use the singular only when talking of exactly one mile, although the real answer may depend on how you say it. The most common usage is "0.25 miles" but a "quarter of a mile".
The generally agreed rule is that you should use the singular only when talking of exactly one mile, although the real answer may depend on how you say it. You use the singular only when talking of exactly  one mile . You'd say "nought point two five miles" (plural) but "a quarter of a mile" (singular). The plural  form is often used for zero persons or  things,  for fractional things in a  quantity greater than one , and for  people or things when the quantity is  unknown.   
One might say "nought point two five miles" (plural) but "a quarter of a mile" (singular). We usually say "0.25 miles" rather than "0.25 mile". The answer to this question is subjective, but it appears that the plural form is much more common than the singular. Some people may advocate only using the singular form for the number 1 and using the plural in all other cases.
Use the singular only when talking of exactly one mile . It depends on how it is spoken - You'd say "nought point two five miles" (plural) but "a quarter of a mile" (singular). Leave singular unless talking about a range of distances. We usually say "0.25 miles" rather than "0.25 mile".
You use the singular only when talking of exactly  one mile . You'd say "nought point two five miles" (plural) but "a quarter of a mile" (singular) The plural  form is often used for zero persons or  things,  for fractional things in a  quantity greater than one , and for  people or things when the quantity is  unknown.   
The use of "0.25 miles" appears to be much more common than "0.25 mile". Some people may say "0.25 mile". The answer to this question is subjective. Some people may advocate only using the singular form when talking specifically about the natural number 1 and using the plural in all other cases.