The industry standard is 3.5 grams for an eighth. Therefore, half an eighth is 1.75 grams. It would be about a half-finger thick in a sandwich baggie if you were getting real small nugs or shake.

Really, though your best bet is to buy a scale; the cheap ones only cost like 5 bucks and are real small. Just don't forget, a folding-sandwich baggie weighs 1 gram and a ziploc sandwich baggie weigh 1.25-1.5 grams, depending on the manufacturer.

Good luck!