What is the correct usage of phrase "you don't know what you don't know"? Can it be used in formal conversation/writing?

8 Answers 8


When expressing frustration or exasperation, we often begin by saying something like, "If I had only known..."

  • If I had known your plane was going to be two hours late, I wouldn't have rushed to the airport.
  • If I had known you were going to get such bad grades on your report card, I wouldn't have let you go to the dance last weekend.
  • Had I known the van was going to overheat, I would have driven the car instead.

Yet, in all these instances, life offers no crystal ball; there's no way of knowing what the future holds, or to be cognizant of something we are unaware of. This is the essence of "You don't know what you don't know," only it's being expressed in a comical way, much like Yogi Berra might have said.

(To those unfamiliar with the Yogi Berra reference, he was a professional baseball player who was reknowned for his wry way of saying things, such as, "It ain't over 'til it's over," and, "You can observe a lot by watching." That said, you couldn't know what you didn't know, but now you know.)

Generally speaking, it would not be a good way to express something in formal writing, unless perhaps you were deliberately injecting humor.

  • 1
    Of course, Yogi Berra said "I didn't say a lot of the things that I said".
    – Jon Hanna
    Jan 29, 2013 at 10:54
  • Incidentally, I'd add that it can also be used in looking toward the future, in awareness that we lack such a crystal ball: We base our plans on our knowledge, and there'll be times where we know there is a gap in that knowledge, but we're also aware that there may be things we can't possibly foresee, because "we don't know what we don't know".
    – Jon Hanna
    Jan 29, 2013 at 10:59
  • @JonHanna: Some risk management techniques urge practitioners to consider the "known unknowns" and the "unknown unknowns".
    – J.R.
    Jan 29, 2013 at 13:43
  • 1
    +1 Great comment! Just Googled Yogi Berra - absolute gold. Yogi also said that "Half the lies they tell about me aren't true"...! On a serious note "Wisest is she who knows she does not know".
    – mdg
    Mar 25, 2013 at 8:21

Socrates said 'I know that I know nothing', which seems to cover the sentiment. I'd say his version is worthy of formal writing or conversation.

  • @Gnawme - +1 (in spirit) for the edit. Feb 25, 2012 at 0:51
  • 4
    Socrates' dictum may be wise, but I think there's a world of difference between OP's acknowledgement that the amount of things one doesn't know is indeterminate, and Socrates' assertion that, for him, everything falls into that category. Feb 25, 2012 at 2:51
  • Awareness (of one's ignorance) is everything but blissful ignorance (unawareness) is surely more inline with the OP. Jan 20, 2017 at 5:00

One of several sources of this and similar phrases is a speech by US Secretary of Defense Donald Rumsfeld on February 12, 2002:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

Whether "unknown unknowns" can be considered "formal" or not in the strictest sense, it is, thanks to this speech, a familiar phrase, reasonably well-understood, to folks in the US who are older than maybe 30 years of age.


It has two very different meanings, that you would have to distinguish by context.

One meaning is just expressing that we have limitations. If you don't know something, that's just tough, you don't know it and you have to live with that. You don't have information if you don't have that information.

The other meaning is that not only are there gaps in our knowledge, but often we don't even know what the gaps in our knowledge are. I don't know how to speak Finnish. That's a gap in my knowledge that I know about. I know that I don't know how to speak Finnish. But there are gaps in my knowledge that I'm not even aware of. That's where you can say "You don't know what you don't know" meaning that you don't even know what knowledge you are missing.


I wouldn't necessarily go as far as call this a "phrase" per se. For me it's a confusing sentence; ambiguous at least.

Whether you can use it or not depends solely on who your audience is. If you're sure the person you're talking to is bound to understand what you're trying to say, then I don't see any problem with using the phrase.

However, if you're unsure the people you're talking to might get confused by the sentence, then definitely don't use it.

Why would you say such a thing anyway? Are you trying to look professional or intelligent? Then use statements which are neither ambiguous, nor difficult to understand in any other way.


I haven't heard or read this expression often enough. But the meaning of 'you don't know what you don't know' is clear to me without being ambiguous. However, I feel it may sound a bit rude to some people and so, I wouldn't want to use it in formal conversations or writing unless I know well the person I am saying it to.

  • Whilst I understand that the meaning may be clear to you "without being ambiguous", this doesn't exactly explain what 'you don't know what you don't know' means, which is what the question asked. I'm also not entirely sure what it means; it could be meant in the sense that you are literally unaware of what you don't know, or it could be meant in the sense that if you don't know something, you don't know said thing. (I do now think I was harsh in downvoting, as you did cover the formal usage part, but my vote is locked in) Feb 4, 2013 at 18:24
  • It is actually quite ambiguous, with two possible meanings that are quite different.
    – gnasher729
    Oct 6, 2014 at 8:55

This is similar to "you know what you don't know".

I think this expression is used when someone is making statements as if fact when they are just opposite. For example, someone speaking about Darwinism old school as a fact. The one speaking knows about something about theory without proof. This is "you know what you don't know".

  • 1
    How is that similar?
    – Jon Hanna
    Jan 29, 2013 at 10:59

It was explained to me along these lines. Each of us has different (spheres of) knowledge, however:

  1. we all know that we know some things (whatever they may be, e.g. "if I tie my shoelaces this way they usually stay done up");
  2. we all know that we don't know some other things (whatever they may be, e.g. for some of us what are the best brain surgery procedures); and lastly,
  3. there are things that each of us don't know that we don't know (I take this to mean they are completely off each of our radars) — and whatever is in this 'sphere' differs from person to person (along with the other two).
  • And what of the phrase "you don't know what you don't know"'s formality? Mar 25, 2013 at 12:03

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