For example:

Once in a while

Now and then

Dictionaries don't provide labeling for every phrase in terms of whether it's formal, informal, slang, etc.

2 Answers 2


If you look up the word occasionally in Longman Dictionary of Contemporary English, it has "S2" next to it, and if you hover over that, it explains that this means it is one of the top 2,000 most common words in spoken English.

Although Longman also defines the phrases "now and then" and "once in a while", it unfortunately doesn't attempt to indicate their frequency.

The best known way to find how much a word or phrase is used is to search for the phrase on Google NGrams. However, this only searches written English, and includes formal texts.

There are searchable corpora of spoken English as well as written, and Longman use these to calculate their statistics. The British National Corpus and the Corpus of Contemporary American English each contain large amounts of spoken material (transcribed and searchable). At https://www.english-corpora.org/coca/ (login required, but free), you can select 'Search' and 'Chart' to see the frequency by "section". For example, in the SPOK (spoken word) section of COCA, "now and then" has a frequency of 6.18 per mil, while "once in a while" is 7.52 per mil - not a great deal higher, to my mind. But for most learners, doing corpus searches is unlikely to be worth while (and it would be easy to make mistakes doing so, for example where a phrase has multiple meanings); plus, most of the sites dedicated to this are liable to prompt you periodically to upgrade to a paid account.

You mentioned that:

Dictionaries don't provide labeling for every phrase in terms of whether it's formal, informal, slang, etc.

But generally speaking, if a word or phrase isn't labelled, then you can assume it is in general use and is neither particularly formal nor particularly informal. Of course, there are exceptions to this. The main one is Merriam-Webster: even when it comes to words (let alone phrases), M-W is extraordinarily sparse with its usage indicators. For instance, it doesn't note that "pinch" (in the sense of "steal") is informal, nor that the phrase "muck around" is informal, nor that "summer" (in the sense of "year") is literary. Lexico, Cambridge, and Macmillan are much better in this respect, as is M-W's Learner's Dictionary, interestingly.

Still, even the better dictionaries don't tell you everything. Hence "it's" isn't marked as informal in Lexico, and yet most people would probably argue for its complete exclusion from formal essays.


Once in while refers to something you do occasionally, but not regularly.


Once in a while I go hiking.

Now and then refers to something you do occasionally, but periodically.


We go to the movies now and then, but not as often as we used to.

Hope this helps.

  • @rose : online dictionaries do mention whether a phrases is formal or informal ( Macmillan , the free dictionary ,ect.)
    – EngFan
    Feb 3, 2017 at 9:13

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