I say it's completely full - half with water and half with air!
Or conversely, it is much less than one percent full because even all the atoms themselves are 99.9999 percent empty space... what is this outlook called? Physicist?
Our local student ASME chapter once had T-shirts that said "To the optimist, the glass is half full. To the pessimist, the glass is half empty. To the engineer, the glass is twice as big as it needs to be and is therefore only 50% efficient."