Q. When writing units of measurement for fractions between 0 and 1, is the unit singular or plural? For example, “We walked 1/4 mile yesterday and 7/8 mile today,” or “Gently fold in 2/3 cup of blueberries.”
A. Amounts of less than one can usually be treated as singular when expressed as simple fractions—as in your three examples—but plural when expressed as decimal fractions. The difference is related to how such expressions would be spelled out or read aloud:
1/4 mile = one-fourth of a mile or a quarter of a mile or a quarter mile (among other variations)
7/8 mile = seven-eighths of a mile
2/3 cup = two-thirds of a cup
A decimal fraction, by contrast, would normally be read as a plural:
0.25 miles = (zero) point two five miles (rather than twenty-five hundredths of a mile)
But note that an abbreviated unit of measure is usually the same for both singular and plural quantities. So you’d write “0.25 mi.” even though “mi.” would be read as “miles.” See also CMOS 9.21 and 10.73.