“A hundred percent” vs. “hundred percent”

Question

Which sentence is grammatically correct:

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

Any help would be greatly appreciated!

Answer 1

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?]

Answer 2

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.