According to OALD both "all at once" and "at once" can mean at the same time.

I can't do everything all at once

I can't do two things at once.

Don't all speak at once!

These examples give me the idea that "at once" could be used when referring to a countable thing (in the latter example all is countable because it refers to a specific count of people, e.g. a class) while "all at once" is taken when referring to a non-countable thing (of course, everything could possibly be a countable thing).

Are they always used interchangeably or is one preferred in certain contexts?

2 Answers 2


There are two different idioms here, and you may be confused. In Don't all speak at once, all and at once are separate, and both modify speak; the normal rules apply to all/both, as Barrie has said. But all at once can also be a single phrase, where all intensifies at once ("When all at once my heart took flight", My Fair Lady). I don't believe there are any rules for when you can or can't use that.


My intuition is to say that you don't need all when you're talking about only two things. The number isn't large enough to justify its use.

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