There is a similar question regarding capitalizing units of measure, but mine is for titles, specifically. For general applications (ie; not inside titles), the consensus seemed to be no for most units (although there were a few special cases mentioned). I am wondering whether this changes for titles. And if so, how it changes. Should you capitalize the first letter? The entire thing?

My specific example is for pounds per square inch (psi). "Tire psi Below Minimum" vs. "Tire PSI Below Minimum" vs. "Tire Psi Below Minimum".

Edit: The commentator below raises a good point. But what if it's "Tire Below 72 PSI Requirement"? I don't see a way to get around using a unit there.

More info: These titles are used as alert titles to display to a user in an application.

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    It's not really a syntactically "valid" usage in the first place, since psi is effectively a unit of measure (akin to miles, hours, kilograms, etc.). In the kind of "shorthand" people might use in documentation, etc., it's reasonably common, but you wouldn't say "My application to become a police officer was rejected because my centimetres are below minimum" (as in there's a minimum height requirement, and I wasn't tall enough). In OP's context it should probably be pressure, not psi. Commented Aug 27, 2014 at 19:03
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    That is a good point - I hadn't thought about it like that. But see my edit.
    – EF0
    Commented Aug 27, 2014 at 19:10
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    Acronyms are typically always capitalized unless the acronym itself has entered the language as a word of its own (e.g. scuba). So I would say use "PSI".
    – TylerH
    Commented Aug 27, 2014 at 19:13
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    @TylerH: In this specific case, (Psi Below Minimum) that exact text only occurs 7 times in the whole of Google Books. But every one of them uses lower case. Commented Aug 27, 2014 at 20:49
  • @TylerH: Do you mean in general or in titles specifically? For in general, I do think it's lowercase.
    – EF0
    Commented Aug 27, 2014 at 21:32

1 Answer 1


(Part of) the answer is actually in the related topic. From the accepted answer:

Unit names are normally printed in roman (upright) type, and they are treated like ordinary nouns. In English, the names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalized material such as a title.

Note that this is about writing the units fully! So pounds per square inch could be "Pounds per Square Inch" in a title.

For the abbreviated forms the standard rule is that the abbreviated form is capitalized is the unit is named after a person. I believe neither the pound nor the inch were named after people (sorry Ezra), so it would be psi. (Strictly speaking, ppsi, p/si or pi-2 would be even better, but they are not SI anyway). There are some other units and derivations that are capitalized for other reasons, by the way (B = byte, b = bit; m = milli, M = mega).

Capitalizing abbreviated units that are normally not capitalized is generally a bad idea, because there are too many cases where you actually change the unit. The unit p could be read as the P for the quantity pressure, for instance, instead if the unit pound. So if you use psi, I would not capitalize it, even in a title.

  • I did read that sentence in the related topic, and it did stick out to me, but in the end I found it unclear. I didn't pick up on it referring to the units spelled out like you are suggesting, and so I was imagining it referring to the abbreviation, and then was unclear about Psi vs PSI. Spelling out the full unit name is not an option in this instance however due to limited space. Your final point about the abbreviation does make a lot of sense though. It makes sense that if changing the case would sometimes change the meaning, it should be avoided altogether.
    – EF0
    Commented Aug 27, 2014 at 21:43

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