Contractions – “you’re”, “we’ll”, “can’t”, “don’t” etc. – generally make language sound less formal, and avoiding them makes it more formal.

Over the years, contractions have become more acceptable higher up the formality spectrum. This is part of a general shift that’s been going on for decades: styles of language that were once firmly seen as casual are now more widely used in more businesslike contexts. Likewise, styles of language that would have been common and neutral in, say, the 1950s now tend to come across as very formal.

On the whole, your best bet is to trust your judgement. Use contractions or not depending on whether you feel comfortable saying the phrase that way, in that sentence, in that context, for that audience. But take care: if your tastes are more old-fashioned or new-fangled than your audience’s, you may miss your mark.

And if you’d like some evidence to double-check your judgement against, I have plenty – from COCA, the Corpus of Contemporary American English.

COCA is a database of English use – 450 million words’ worth – from a wide range of sources. It covers fiction, newspapers, magazines, academic journals, and transcripts of unscripted TV and radio, from 1990 to 2012.

I searched COCA for 77 contractions and their spelt-out counterparts (there are others, but life is short). For instance, I searched for “didn’t” and “did not”, recorded the number of uses per million words, and then divided the “didn’t” number by the “did not” number.

This gives the relative frequency of use for each contraction: how common each is relative to its spelt-out version. A small number means a phrase is rarely contracted, 1 means both versions are equally common, and more than 1 means it’s usually contracted. The more common a contraction is, the more comfortable you can feel about using it.

Before we get into the detail, here are two top-level findings, averaged across all 77 contractions. First, the trend over time:

Over just a couple of decades, contraction use has increased.

Second, the differences between different kinds of source (averaged across 1990–2012):

No surprises here. Contractions are extremely common in speech and fiction, quite common in magazines and newspapers, and less common in academic journals.

The next tables, getting into the detail, only cover non-fiction writing. This is because I think people are generally happy judging when to contract phrases in speech (and fiction, for those who write it). So from here on, I’m only looking at newspapers and magazines (averaged together, as their overall results are similar) and at academic journals.

Here are contractions ending in “–n’t”:

The lower the frequency, the more cautious it’s wise to be. But even for common contractions, there will be sentences where it’s better to spell them out. And for rarer contractions, there will be sentences where they work better.

The other common kind of contraction is the sort that joins a pronoun and a verb of the “be” or “have” families or “will”, “would” or “had”.

Here it gets a bit tricky:

“Would” and “had”. “I’d” can mean “I would” or “I had”. So to get meaningful results, I had to search for slightly longer phrases. I compared “I would be” with “I’d be” and “I had been” with “I’d been”. Likewise for other pronouns.

“I’d” can mean “I would” or “I had”. So to get meaningful results, I had to search for slightly longer phrases. I compared “I would be” with “I’d be” and “I had been” with “I’d been”. Likewise for other pronouns. “Is” and “has”. “It’s” can mean “it is” or “it has”. So I compared “it is being” with “it’s being” and “it has been” with “it’s been”. Likewise for the pronouns “he”, “she”, “who” and “that”. This doesn’t work for “there”, because “there is being” doesn’t really get any use. So I compared “there’s been” with “there has been” and “there is a” with “there’s a” (“there has a” is pretty much non-existent).

These are the results:

“Be” verb phrases are the ones most often contracted. In newspapers and magazines, the other kinds (especially “will”) are also often contracted. “Is” is contracted more often than “has”, and “would” is contracted more often than “had”. In journals the pattern is mostly similar but the numbers are smaller.

In both tables, “you” phrases are the most often contracted, followed by “I” and “we”. First-person writing tends to be more casual, and writing that addresses the reader in the second person even more so. Phrases using other personal pronouns – “he”, “she” and “they” – are next-most-often contracted. “It” and “there” phrases follow, although most of the contractions there are “it’s” and “there’s”. Bringing up the rear are “who” and “that” phrases (with a strong showing from “that’s”).

I looked at a few other contractions that don’t fit either of the above groups.

“Should’ve”, “would’ve” and “could’ve” are pretty rare, with relative frequencies of 0.02–0.03 in newspapers and magazines, and 0.00 in journals. “Let’s” (for “let us”) is very common, scoring 3.79 in newspapers and magazines and 0.70 in journals.

I’ll end by quoting some recent usage manual and style guides with advice that, in light of this data, seems fair:

Contractions of the type I’m (= I am) and don’t (= do not) are exceedingly common in informal and online writing and increasingly found in various kinds of fairly formal contexts (e.g. in book reviews). – Fowler’s Dictionary of Modern English Usage (2015)

In the past they were felt to be too colloquial for the written medium, and editors of academic journals are still inclined to edit them out. The writers of formal documents may feel that they undermine the authority and dignity of their words. But the interactive quality that contractions lend to a style is these days often sought, in business and elsewhere. They facilitate reading by reducing the space taken up by predictable elements of the verb phrase, and help to establish the underlying rhythms of prose. – Cambridge Guide to English Usage (2004)

Many writers, especially those who write in formal situations, feel uncomfortable with contractions. And perhaps contractions don’t generally belong in solemn contexts. But why shouldn’t writers use them in most types of writing? Some excellent writers use contractions to good effect, even in books… The common fear is that using contractions can make the writing seem breezy. For most of us, though, that risk is nil. What you gain should be a relaxed manner – not breeziness. – Garner’s Modern American Usage (2009)

Sometimes, lots of ‘cannot’, ‘should not’ etc can seem archaic and formal. That’s a tone we can move away from without jeopardising the overall tone of information coming from government. – Writing for GOV.UK (2015)

Most types of writing benefit from the use of contractions. If used thoughtfully, contractions in prose sound natural and relaxed and make reading more enjoyable. Be-verbs and most of the auxiliary verbs are contracted when followed by not: are–aren’t; was–wasn’t; cannot–can’t; could not–couldn’t; do not–don’t; and so on. A few, such as ought not–oughtn’t, look or sound awkward and are best avoided. Pronouns can be contracted with auxiliaries, forms of have, and some be-verbs. Think before using one of the less common contractions, which often don’t work well in prose, except perhaps in dialogue or quotations. Some examples are I’d’ve (I would have), she’d’ve (she would have), it’d (it would), should’ve (should have), there’re (there are), who’re (who are), and would’ve (would have). Also, some contracted forms can have more than one meaning. For instance, there’s may be there is or there has, and I’d may be I had or I would. The particular meaning may not always be clear from the context. – Chicago Manual of Style (2010)