We use the numerical version at work all the time. We write it as such: 3.5"; 3.50"; 3.500". Although all the dimensions are the same the measurement tolerance changes with the addition of the zeros. We never write "inch" or "inches"; we always use the " sign.
When we use fractional measurements like 1/2" or 1/4" the tolerances are usually in fractions and are usually much larger tolerances than the numerical ones. So the choice of numerical or fractional is not a casual choice.
Are you writing or speaking it?
I would expect to see 0.5" or 0.5 in. in informal writing - the symbol or abbreviated form is common, and people often can't be bothered to put in ½ .
In speech, I would expect people to say 'half an inch' (even if written 0.5") most of the time, but if they say it in decimals, then it would be '(zero/nought/oh) point five inches'.
If you want the English to be just like what they will encounter in written English outside the text you are writing, I recommend you use the normal, conventional way that nat suggested, of 0.5 inches.Considering all of the above, shall I write 0.5 inch or 0.5 inches?
I am trying to use the inch word instead of " to denote "inch".
Considering all of the above, shall I write 0.5 inch or 0.5 inches?
but if they say it in decimals, then it would be '(zero/nought/oh) point five inches'.
Sorry if I wasn't clear on what I meant.In fact what nat recommended was 0.5" or 0.5 in. He put a period after 'in'. I think it would be better to omit the period. Feet, by the way, are designated by a single quote or 'ft'. 0.5' or 0.5 ft is half a foot. For people accustomed to using the metric system, it may not be obvious that double quotes means inches. I think that 'in' and 'ft' may be your best choice.