They mean exactly the same thing — cos is just a clipped, spoken form of because: I left early because the trains were cancelled versus left early cos the trains were off. Cos belongs in speech, texts and dialogue; because is what you write everywhere else. There is no difference in meaning at all, only in register.
The core difference
Nobody says all three syllables of because in relaxed speech. What comes out is closer to cuz or c'z, and English has developed several written spellings for that reduced form. Cos is the usual British spelling. 'Cause and cuz are more common in American writing. All of them are the same word, informally dressed.
Where cos is fine
- Text messages and chat: can't make it cos I'm still at work
- Dialogue in fiction, where it signals how someone actually talks: "I only asked cos she never says anything."
- Transcripts and song lyrics, where fidelity to speech is the point.
- Deliberately casual brand or social copy — used sparingly.
Where you need because
Essays, reports, emails to anyone you do not know well, applications, anything published under your name. Cos in a cover letter reads as carelessness rather than warmth, and readers who do not recognise it may briefly parse it as the trigonometric abbreviation for cosine.
The test
Ask: would I be comfortable saying this sentence out loud to the reader, in these exact words? If the whole message is written the way you speak, cos fits. If a single cos would sit among otherwise careful sentences, it will look like a typo. Register has to be consistent — that is the real rule, and it is more useful than banning the word outright.
Spelling the informal form
If you do use it, note that cos takes no apostrophe in standard British practice, while American style usually writes 'cause with a leading apostrophe marking the missing syllable. Coz is a further variant, and in some contexts a shortening of cousin instead. Pick one form and stick to it.
The related trap
There is also the internet construction because reasons or because science — because followed straight by a noun, with no clause. It is a genuine, widely understood joke construction, and it is knowingly ungrammatical. It works in casual writing precisely because everyone can hear the missing words; it has no place in formal prose.
One more: because of takes a noun phrase (because of the rain), while plain because takes a full clause (because it was raining). Mixing them — because of it was raining — is a genuine error, unlike cos, which is merely informal.
