What is the difference between these two sentences:

  • I’m one hundred percent sure.

  • I’m a hundred percent sure.

For me the only difference between the two sentences is that the second sentence is more formal.

  • Related: ell.stackexchange.com/questions/923/…
    – J.R.
    Jan 10, 2015 at 15:18
  • The former is a little more common, but have roughly the same meaning.
    – Tim S.
    Jan 10, 2015 at 15:46
  • Wow, I'm surprised; thanks for that, Tim. Subjectively I felt that “a hundred percent” would be more common. But even when I removed the “sure” and limited to British or American English, “one hundred percent” came on top. I may need to reconsider my answer.
    – Celery Man
    Jan 10, 2015 at 17:01

4 Answers 4


The two phrases have the same basic meaning, but native speakers tend to use the "one hundred percent sure" wording in order to to give a greater impression of certainty. In speech, you can emphasise the word "one" more strongly than "a" and the phrase is spoken with a regular rhythm as ONE HUNdred perCENT SURE.


There is no difference. However, I have heard the former more frequently. Maybe, in India though.

OALD defines it:

a/one hundred percent - in every way.

  • Thanks Maulik; but I heard using 'one' in this sense is nit common at least as using the determiner 'a'. Have you ever heard or read something about it somewhere? :-/
    – A-friend
    Jan 10, 2015 at 10:03
  • Nima, I searched. Both are in use. Right now I'm away and on mobile. May get back to you.
    – Maulik V
    Jan 10, 2015 at 16:06

“One hundred percent” is more emphatic. When spoken, the stress naturally falls on “one”. But other than that, they are completely equivalent. A similar emphasis can be achieved with “a hundred percent” by stressing the quantity “hundred”.

Note the examples given in Oxford Learners Dictionaries:

  • I'm not a hundred per cent sure.
  • My family supports me one hundred per cent.
  • I still don't feel a hundred per cent.

The statement about having family support is stronger than the other two.

“I still don't feel one hundred percent” is not incorrect, but feels more awkward when spoken. A native speaker would more likely use “a hundred” here, as in the dictionary example. Perhaps “don't feel a hundred percent (well)” means “don't feel quite well”; but “don't feel one hundred percent” or “don't feel a hundred percent” mean “don't feel completely well”?

“Not 100% sure” is an interesting case. Because of the negation, you would normally use the softer “a hundred” form. To me, “I'm not a hundred percent sure” means “I'm not very sure”; whereas “I'm not one hundred percent sure” means “I'm reasonably sure but not completely certain” (with the spoken stress on the words in italic/oblique font).

This is not a distinction that I see in the affirmative “I am a/one hundred percent sure”. Both mean “completely sure”. It's only a difference in emphasis, not degree.

There's also a third written form that I've slipped in above: “100%”. Here, 100 can be pronounced a hundred or one hundred, and the writer is giving no indication as to which.

[Acknowledgement to Maulik and David, I've drawn on their answers in formulating this one.]


I tend to say “I’m a hundred . . .” but that may be because of my Southern U.S. tendency to slur over certain word landscapes, so to speak.

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