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I've seen people using the phrase once a year but I wonder isn't it should be once in a year. Are they both the same and acceptable?

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  • Regardless of correctness this would probably identity you as a non native English speaker
    – Stevetech
    Sep 15, 2016 at 7:50

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There is a slight, but meaningful, difference.

"Once a [time period]" implies frequency observed over a range of time greater than [time period]. "Once in a [time period]" implies an observed frequency of 1 in a single [time period].

If somebody were to say something occurred once in a month then he means he is talking about an event with a time of a month. If someone says something occurs once a month then he definitely has data larger than a month.

Example:

My last boyfriend, Eric, would buy me flowers once a month! But John has only bought me flowers once in a year. Does John hate me or is he just less sentimental and thus will only buy flowers for year anniversaries?

Another example:

During the first year of our relationship, Phillip bought me flowers once a month. Why would he now only buy me flowers once in a year?

As FumbleFingers brings up in the comments, the "once in a [time period]" could instead be an application of figurative speech and not based on technical data. The following example shows a usage of the phrase that indicates exaggerated speech is at play.

Wow! That's rare; it only happens once in a blue moon!

This is perhaps more indicative of a common idiom but still important to bring up.

To slightly elaborate on what was established above, it may not be obvious that the phrase "once in a [time period]" can be used to establish a cap on repetitions in a [time period]. See below.

Alice: "So, he goes to Mexico 10 times a year?"

Bob: "Yes, but never twice in the same month; he only goes once in a month."

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  • The most common instances of once in a [time period] are probably blue moon, lifetime, century - which all reflect the same rarely implication as your own example, so I think that's relevant to the matter of contexts where in works. I agree it's very often used in contexts where in a [time period] effectively means since a [timeperiod] ago (until now). But there's always an allusion to "frequency" - which is indeterminate from a sample of one, so the "imprecise measurement" allusions are right there just under the surface.
    – FumbleFingers
    Sep 13, 2016 at 18:27
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    Hmm.. reading it in the question, my instinct for "in a" was a future tense sentence - "I'd only go on vacation once in a year", something that would occur more than one time, but not necessarily every year. The past-tense example at the bottom of your answer sounds really awkward.
    – Izkata
    Sep 13, 2016 at 18:33
  • FumbleFingers and Izkata, are there usages of what you are bringing up in which you cannot remove the word "in" and it still make sense? The only one I could think dealt with an idiom (once in a blue moon).
    – Harrichael
    Sep 13, 2016 at 19:57
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    +1 this is hard to explain at common usage context, but you did elaborate it well on the last part. That being said, it's noticeable that even in formal technical context, in can be remove/omit if [time period] is referring to a collective time noun(I don't know what should I call it), and in should be implicit if/while using a range of [time period] like, twice in the same month. This clearly sums it up.
    – Aesthetic
    Sep 14, 2016 at 15:23
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    Why the addition of "max"? Only already restricts the meaning; it's more likely you would have one or the other. Plus "max" is informal for "a maximum"
    – eques
    Sep 15, 2016 at 15:44
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The main reason they're "not the same" is because of this huge difference in frequency of use...

enter image description here

I don't think there are any many1 contexts where you need to include in, so the easiest option is never to use it (because in most contexts it will sound odd to native speakers if you include in).

But as often happens in English, where one of two different versions of an expression becomes dominant, the less-favoured alternative may nevertheless survive, and start to carve out its own "semantic territory" and associations. Take, for example...

1: It doesn't snow here often. Usually not more than once in a year.
2: She doesn't go out much. Probably not once in a month.

There's nothing "unidiomatic" about those usages (though they'd be fine without the extra preposition). I think the reason they're acceptable is because they stress the infrequency of the event (it's rare, uncommon).

Another important factor is that the occurrence rate being referenced has to be somewhat imprecisely measured, unpredictable, irregular. So (for me at least)...

3: I've always had regular checkups. I see my dentist once in a year

...doesn't work at all.

1 Note that in has semantic significance in constructions like I haven't seen him once in a year, where it means once in the past year (i.e. - the twelve months immediately preceding time of utterance). There are also closely-related usages such as once in a lifetime, once in a blue moon where in is idiomatically required. As with the examples above, all these contexts imply infrequent / sporadic.

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    I thought there could be a situation where something happened once in a year, where you have a specific year in mind (like last year), and don't know anything about other years
    – Mario Trucco
    Sep 13, 2016 at 10:27
  • @Mario: Unless the context included both elements above (infrequent and sporadic) I think that would normally be something that happened once in the year [whichever year it was].
    – FumbleFingers
    Sep 13, 2016 at 12:24
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    The first statement seems supported by the second. Both are false. While I agree there are usages where the "in" can be removed and the same meaning retained, this is not true in all instances. See my answer.
    – Harrichael
    Sep 13, 2016 at 19:59
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    I don't think number three is really "doesent work at all", it makes perfect sense to me and I've heard many people say that.
    – Muntasir Alam
    Sep 14, 2016 at 2:55
  • @Muntasir Alam: I have to say that seems odd to me, given my headline chart showing how rarely in is used at all. Obviously my final example "makes sense", in that anyone can understand what was meant. But my specific context explicitly excludes the two main factors (infrequent, sporadic) that are closely associated with those few contexts where in is still likely to be used. Perhaps this is an aspect of usage where Canadian English is a bit out of step with "mainstream Anglophones".
    – FumbleFingers
    Sep 15, 2016 at 16:23
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It's important to remember: the word a can be used as a preposition, meaning per. See this screen shot focusing on just a few of the many definitions found on Wordnik:

enter image description here

Therefore, the phrase once a year is perfectly grammatical, acceptable, and idiomatic.

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Once a year is considerably more common: here is an NGram. It implies that something takes place once in every calendar year. For example:

I take a holiday in Norway once a year

Once in a year is much less common and is in decline: many of the older references relate to annual religious observances. In modern usage, it refers to something that happened only once within a one-year period which need not necessarily be a calendar year, for example:

He bought me a drink only once in a year.

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Once a year/month/week would be more common. Once in a year/month/week would be understood, but less likely used. You are more likely to hear "Once in the past year" or "Once in the past five months" than "once in a year"

For an expression of the form "once a ", "a" acts as a preposition meaning "per"; it could also be defined as "each"

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    It is completely irrelevant which is used more often, and which one you are more likely to hear. What's important is which one is correct to use in which situation.
    – gnasher729
    Sep 12, 2016 at 20:27
  • It actually is not completely irrelevant; it might be secondary. Plenty of questions comment on the relative frequency of occurrences of expressions. "Once in a year" is less common in most situations than "once a year" except for examples like @Michael
    – eques
    Sep 12, 2016 at 20:37
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I think that there's a precision difference between those two phrases.

"Once a year" says to me that every year, about the same time, something happens.

He goes on vacation in Florida once a year.
- It's some sort of a tradition, which is usually tied to a particular time.

He only goes to Florida once in a month.
- It's more rare to hear this, especially longer than a month, and it's a "may or may not happen" sort of occurrence.

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  • Got any sources for "once a year" implying some degree of regularity for when within the time frame it occurs? It doesn't seem to imply that in all cases.
    – eques
    Sep 13, 2016 at 1:12
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    I am not alone in this distinction: forum.wordreference.com/threads/… However, I can't find anything official... Still looking...
    – BenPen
    Sep 13, 2016 at 4:52
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    Annual and "Once a year" are synonyms, and annual is definitely a periodic occurrence roughly a year apart from the last time it happened: thesaurus.com/browse/annual. But, in general you can see from usage, most, though not 100% of the time, "once in a year" is describing an at least sort of event, where "once a year" is a schedule.
    – BenPen
    Sep 13, 2016 at 5:13
  • I see nothing in your link to suggest that "annual" means "around the same time". "I take an annual trip to London" means I take a trip to London once a year, but isn't indicated that it is the same time each year
    – eques
    Sep 13, 2016 at 14:08
  • Maybe it's too much of an inference, but I would say that an annual trip or an annual contest, annual renewal or whatnot is around the same time each year. When you are discussing trips, for instance, I usually hear a clarification, "I'm taking my annual trip to Europe, but it's in April this year." Highly nuanced, but the examples in dictionary.com's definition imply more than the definition says. Even "annual salary" which is sort of a different case, means at the end of a year, you have collected all the money in the annual salary...
    – BenPen
    Sep 13, 2016 at 15:53
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In addition to what has already been explained, it would be presumed that an event which happens once a year probably occurs around the same time. Valentine's Day only comes once a year.

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"Once a year" denotes something recurrent that happens each period.

"Once in a year" denotes something happened in the last year.

"I used to party once a week, but since I married I went to a party once in a year."

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  • Not necessarily in the past year; it could refer to some other timeframe. For example: "When I was recovering from my surgery, I only went to a party once in a year." That usage might be considered a shortened form of "once in a year's time."
    – J.R. ♦
    Sep 13, 2016 at 21:12
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From my perspective this is the difference in meaning:

  1. Once in a year: The expectation is that it happens each year and 1 time each year
  2. Once a year: The expectation is that it happens at a 1 year interval

Example

This difference is usually not relevant, but here is an example where only 1 of the two seems appropriate:

Someone whose birthday is around 1 january, may celebrate it in december or january. In this case it would be strange (wrong?!) for the person to say:

Once in a year I celebrate my birthday

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