Idiomatically, a quantity is singular if and only if it is being regarded as a single entity (including a fractional part of a whole) and is not in decimal form:
1 litre is left
¾ (of a) litre is left
three-quarters of the cake is left.
five-fourths of the cake is left.
1.0 litres (e.g., 1.003 litres to 1 decimal place) are left
one point zero zero dollars
0.75 litres are left
0 litres are left
1¾ litres are left
three-quarters of the pieces of cake are left
three quarters of the cake (three quarter-cake slices) are left.
This convention for quantities, which are by definition nonnegative, holds regardless of whether the unary operation ‘minus’ is being applied. So, “-1 litre” and “-0.75 litres” (whatever these mean) are correct.
When a plural noun is being regarded collectively, it agrees with a singular verb:
- 3 dollars is a nominal fee
- 2.5 litres is not a lot.
The choice between “half the crew is late” and “half the crew are late” depends on the intended emphasis.
Note that “I have no legs” is alluding to the number 0 (as opposed to specifying 1 leg or 2 legs or any other number of legs), whereas “I have no leg” is noting the absence of a leg (i.e., saying that I have not one leg).
