you can't judge a book by its cover

don't judge a book by its cover

Don't base your opinion of something (or someone) on the way it (or one) looks. This report may look dull, but don't judge a book by its cover—I will have you riveted by the facts and figures in no time! I know she dresses a little frumpy, but don't judge a book by its cover, man!
See also: book, by, cover, judge

you can't judge a book by its cover

proverb You cannot determine or should not judge the true qualities or characteristics of someone or something just by assessing superficial details alone. He seemed like a bit of a jerk to me, but I guess you can't judge a book by its cover.
See also: book, by, cover, judge
Farlex Dictionary of Idioms.

judge a book by its cover, one can't

One can't rely on outward appearances to know what something or someone is really like. For example, He seems very quiet, but you can't judge a book by its cover. [First half of 1900s]
See also: book, by, judge, one
The American Heritage® Dictionary of Idioms by Christine Ammer.

you can't judge a book by its cover

or

you can't judge a book by the cover

You say you can't judge a book by its cover or you can't judge a book by the cover to mean that you should not judge someone or something by what they look like or what they seem like at first. If he loves her I'm happy for them. You can't judge a book by its cover. We may say that we don't believe in judging a book by the cover, but research has shown that we do, over and over again.
See also: book, by, cover, judge
Collins COBUILD Idioms Dictionary, 3rd ed.

don’t judge a ˌbook by its ˈcover

(saying) used to say that you should not form an opinion about somebody/something from their appearance only: When we arrived we found that the hotel we’d booked looked awful, but as they say, you should never judge a book by its cover.
See also: book, by, cover, judge
Farlex Partner Idioms Dictionary
See also:
  • Don’t make a federal case out of it!
  • don't bet on it
  • don't get me wrong
  • don't beat a dead horse
  • all in (one's) head
  • don't cry before you are hurt
  • a thing you don't want is dear at any price
  • come it
  • come it (with one)
References in periodicals archive
As the saying goes, you can't judge a book by its cover. At first blush, this book appears to be about baseball, but really, it's about value investing.
Hit the books: four terrific picks to hello you learn more about investing
The old saying is true: sometimes, you can't judge a book by its cover. In this case, you can't ignore reading the fine print of legislation.
Reading "between the lines" of new legislation
As a young reader, I learned the cliche "You can't judge a book by its cover."
10 Tongues
If there's any lesson the Barbis would have you learn, it's that you can't judge a book by its cover.
Twin peeks
Take, for instance, "You can't judge a book by its cover." The book in hand is Financing the American Dream: A Cultural History of Consumer Credit, by Lendol Calder, and, although it is a revision of a doctoral dissertation, it reads like a work of seasoned scholarship.
Charge It!
We've all heard that you can't judge a book by its cover. But that doesn't mean you can't judge it by its height and weight.
The book on Koen's weighing and cubing system
You can't judge a book by its cover, but can you judge a man by the books he gives to his, er, inamoratas?
Poetic licentiousness: what does the president see in 'Leaves of Grass'?