Per (from a Latin word meaning “by,” “by means of,” or “through”) is widely employed in English, but it — or a phrase in which it appears — isn’t necessarily the best option. Here’s a guide to using (or not using) per.
In popular usage, per is appropriate in relation to figures (“65 miles per hour,” “24 points per game,” “three items per person,” and so on). It can also, alone or preceded by as, mean “according to,” as in “Per your instructions, I deposited the check” and “She complied as per the usual procedure.” Per is also an adverb synonymous with apiece; though its use in this form may seem like dialect in which the final word of the sentence is dropped (“I’ll let you have them for five dollars per”), it’s standard usage.
In its most common function, however, it is sometimes easily replaced by a, as in “The position pays $75,000 a year” rather than “per year” or “I exercise three times a week” rather than “per week.”
Per as part of a set phrase taken directly from Latin is usually pedantic in popular usage; see the list below for recommendations about the use of such phrases:
Per annum: This is an unnecessary substitution for the perfectly useful phrases “a year,” “each year,” or even “per year.”
Per capita: This direct borrowing of the Latin phrase meaning “by the head” is commonly employed in financial and demographic contexts, but “per person” is better in general.
Per centum: The Latin precursor of the semiabbreviation percent more clearly signals the original meaning (“for each hundred”) but is obsolete.
Per contra: This phrase, meaning “in contrast to,” is best reserved for legal contexts — and only technical ones, at that.
Per diem: The translations for this phrase are “by the day” or “for the day,” and except in reference to a daily stipend, there’s no reason not to use daily (especially as an adjective).
Per mill: In this phrase, mill is from the Latin term mille, meaning “a thousand,” so the term is akin to percent but spelled as two words. It’s appropriate only in technical usage, however.
Per se: Outside of legal usage, this term — sometimes misspelled “per say” by those who mistakenly assume that it refers to the act of speaking — means “in itself,” as in “I don’t object to the idea per se.” (Note that, unlike as is the case when the English form is used — “I don’t object to the idea, in itself” — no punctuation frames the term.)