Assuming I don't want to say "zero point four grams to one gram," would it be:
"zero point four to one gram"
or
"zero point four to one grams"
or neither?
I'm leaning towards the second one because it just sounds better and I would argue that "grams" refers to a range of measurements, as opposed to a single measurement (e.g. two grams). Would this be correct?
There is a similar question on Stack Exchange (Is two-thirds plural?). I am not sure that any of the answers to that is definitive. In Quora, there is a reasonable answer, which says:
"As an abstract number, it's singular: “0.5 is larger than 0.4.” As a quantifier of a unit, it's usually plural (at least in technical writing): “0.5 V” is pronounced “0.5 volts” (cf. “1 volt”). (Basically everything except “a”, “an”, “1” or “one” is plural in this context. Even “1.0” is plural.)" In a way this seems counter-intuitive. Surely a fraction of a gram, or volt, or whatever, cannot be plural when we are talking about less that one. Well, yes it can.
One way of looking at it is to think of the question you would ask to find out some measurement of something:
"How many grams/metres/litres/degrees IS it?" And the answer will be: "it weighs 0.4 grams"; or "it is 0.452 metres long"; or "it contains 0.4 litres of wine"; or "it is 0.4 degrees celsius below freezing." The only exception to this rule is the number '1'. Then and only then is the measure expressed in the singular.
So your problem is about what happens when you have both fraction and single whole together? My answer is that this is one of those cases where you reach a limit point and where, therefore, you can legitimately take your pick.
A: ... from 0.4 to 1 gram: this can be read as "... from 0.4 to 1 gram" where the plural measure in angled brackets is left to be understood.
B ...from 0.4 to 1 grams: this can be read as a price range 'from nought-point-four-to-one grams', where plural wins.
My suspicion is that there will be a significant enough proportion of speakers feeling 'uncomfortable' about either of the two. So if you want to be really sure to leave nobody uncomfortable, you need to use the combination you don't want: "from 0.4 grams to 1 gram." Sorry, but there it is.
The question is how would you say the amounts mentioned. Unless compelled to do otherwise I would say, as distinct from write, "between 400 and 1000 milligrams". If you try to say decimal places, the people you speak to will lose count of the numbers, and you will fail to communicate to them what you want them to know.
Just to supplement Tuffy's answer, here is what the Chicago Manual of Style (CMOS) has to say on the subject:
10.53: Plurals for SI units
Though abbreviations for SI units are the same for plural and singular forms, the noun forms for such units would generally be written out or pronounced in the plural (e.g., 3 m = three meters; but a three-meter span). The only exception is for a quantity of exactly 1; for quantities such as 0.5 m or 1.6 m, the unit would generally be read as if it were plural (zero point five meters; one point six meters).
For what it's worth, I agree with Tuffy and Jason Bassford that the only safe option in your case is the one you don't want, i.e.
0.4 grams to 1 gram.
Some 'cheats'
The following I consider cheats because they seek to avoid the very problem that I take to be the essence of your question, which is how to confront head-on the special problems that the exact value of 1 creates.
In particular, none of the cheats can handle a related problem of how to phrase (the admitedly somewhat artifical example of)
the range of minus two units to one unit
without using the word unit (in either the singular or plural form) twice.
In one cheat, one can use the fact that although it is 1 gram, it is nevertheless 1.0 grams (which is implied by the CMOS section I quoted above, and as Tuffy mentioned as well). Then you would say
0.4 to 1.0 grams.
Another cheat is the suggestion by JeremyC to convert to milligrams (400 to 1000 milligrams). Although a cheat by my definition, it is certainly a very good suggestion in practice.
