The Oxford English Dictionary defines "bpm" as an abbreviation of "beats per minute" and gives the example:
a pulse rate of 40 bpm
Is there an explanation for the curious feature that this abbreviation is set in lowercase?
Learn English – Capitalization of bpm/BPM meaning beats per minute
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Best Answer
This is really a question of style. Abbreviations (or, in this case, initialisms) are not necessarily capitalised, especially when all of the words would be spelled out without a capital letter: there's an example in the question where "beats per minute" is spelled out with minuscule letters.
Most initialisms either have a proper noun as at least one of the words, which may cause the whole to be capitalised, or the term is important in the document (eg Wikipedia's example of congestive heart failure [CHF]). It's very unlikely that a term like "beats per minute" needs to be made prominent in a sentence within a text. Setting it in capitals could upset the balance of the typesetting, and lowercase is easier than using small caps BPM.
If it might get lost, for example when filling in a form, it might well be capitalised.
There is an additional set of initialisms — acronyms, which can be pronounced as words — and where these are common nouns they are customarily set in lower-case: scuba, laser, radar, sonar and the like.