Which sentence is grammatically correct:
- I'm a hundred percent sure
- I'm hundred percent sure
Any help would be greatly appreciated!
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?]
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.