To expand a bit more, if the acronym is pronounced as a word, then you use the word’s pronounciation. So one has “a LASER”, not “an LASER”. But if it were spoken as a series of letters, it’d be “an ell-ay-ess-ee-arr”.
And to expand yet a bit further, the pronounciation always rules for all words, not just acronyms. So one has “a unicycle”, since that’s pronounced “yoonicycle”, but “an umbrella”, and “a house”, but “an honor”, since the “h” in honor is silent.
What Chronos said is what I was told when I learned English as a foreign language. I memorized it with the rule that, no matter how it is written, it’s “an” whenever a vowel is spoken after the indefinite article.
There could be some ambiguity. For example “URL.” Now, some people pronounce it as a word, like “Earl” (Weird Earl’s anyone?) In that case, you’d write “an URL.” If you intend it to be said as “U-R-L,” then it’s “a URL.”
Side question: is there a rule on when acronyms are pronounced or spelled? I tend to see (okay, hear) TLAs always spelled out, while four-letter ones say the first letter and pronounce the last three (pee-sat, gee-mat, ell-sat, but gee-arr-ee).
I think the closest thing to a rule is “pronounce it if you can”. I would never refer to digital audio tape as “dee ay tee”, for instance, but as “dat”. Then again, the college entrance exams are the “ess ay tee” and the “ay cee tee”, not the perfectly pronounceable “sat” and the “act”, so that’s really more just a guideline.
In publishing, the Table of Contents is a TOC – usually pronouned “tee-oh-cee.” We used to have a typesetter who called it a “tock,” which annoyed the hell out of me. Alas, she trained her replacement to call it a “tock” also before quitting.
An acronym is always pronounced as a word. “TLA” is an initialism or abbreviation. So, for example, NOAA, NASA, laser, radar, and scuba are acronyms (as well as being initialisms and abbreviations). FBI is not an acronym, unless you happen to pronounce it “Fibby.” There is somedispute on this point, and I fully recognize that the common and popular definition will eventually scalp and slaughter any pedants who choose to make this their Little Big Horn.
I agree that “pronounce it if you can” or perhaps “pronounce it if you can easily” is the rule. And that there is often not a consensus.
Maybe the reason that SAT, ACT, and ID (which isn’t really an acronym anyway) are spelled out is that they are really words (“sat” being the past tense of sit, etc.) Of course, PIN is the exception to this rule, so maybe we should quit trying to explain things with rules, since we’ll never be that accurate anyway.
Also, SAT and ACT spelled out sound almost the same:
Eh Say Tee
Ay See Tee
People that take those tests (teenagers) tend to mumble so that they both sound like “Uh Sitee,” resulting in:
“I’m taking the Uh Sitee this weekend.”
“I thought you were taking the Uh Sitee?”
“Yeah, that’s what I said.”
“Wait, are you taking the Uh Sitee or the Uh Sitee?”
“The Uh Sitee”
I’m ashamed to say that I’m a part of that group, and probably guilty of mumbling, to some extent. But I got a 1550 out of 1600 on my first try, and an 800 out of 800 on both of the subject tests I’ve taken. So I’m better than everyone else. Kinda.