grammatical number - Correct usage of lbs. as in "pounds" of weight

I suggest writing 2.3 kg instead of any of 5 lbs., 5lbs., 5 lb. or 5lb. Historically, however, the forms “5 lb” and “5 lbs” appear to have been used more than either of the others (according to ngrams for 5 lbs,5lbs,5lb,5 lb,5 lb . [where 5 lb . represents 5 lb. ie has a period after lb as part of the search]) :

Note, if you click on the book links on the ngrams page, you will note that in many instances the 5 is after a decimal point, eg, “12.5 lbs.”.

Note, naturally 5 lb occurs more often than other forms because 5 lb occurs in every instance of any of {5 lbs, 5 lb, 5 lb., 5 lbs.}. Ngrams for 5 lbs,5 lb,5 lb .,5 lbs . shows that 5 lb. and 5 lbs. nowadays occur with nearly equal frequency, the latter slightly more frequently. Before 1980, 5 lb. occurred rather more frequently than 5 lbs.