2

Which sentence is grammatically correct:

  • I'm a hundred percent sure
  • I'm hundred percent sure

Any help would be greatly appreciated!

7

Hundred by itself does not constitute a quantity. You must have a number in some form:

I'm one hundred percent sure.

or something else to provide a number — like "a" which denotes a singular quantity.

I'm a hundred percent sure.

Without a number it is similar to saying

I have milliliters of water. [How many milliliters?]

3

One hundred is usually not said in English unless wishing to distinguish it with TWO, for example; in this case, A hundred is the idiomatic way of referring to ONE hundred. And, in agreement with a previously posted answer, without saying A (=ONE), no quantity would be indicated.

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