half eighth (1/8 divided by 2) of marijuana is called a teenth (1/16)

meaning it is 1/16th of an ounce

now lets do the maths!!

1 ounce = 28.349523125 grams (hey i just done you some research.... let's round this up to 28.35g)

28.35g / 16 = 1.77 grams of marijuana

now.. if you have a good dealer or grow your own it will not have any 'additives'. and this may be enough for a couple of spliffs depending what size they are.