What weights the same as marijuana?

An "eighth" refers to 1/8 of an ounce. This is also equivalent to 3.5 grams. There are 28 grams in an ounce (3.5*8=28).

Any other term, like a "quarter" or half" usually refers to the amount of weed out of 28 grams/a full ounce. (e.g. a quarter is 1/4 oz, or 7 grams)

Also, a qp refers to a quarter-pound. This would be 4 ounces, or 112 grams.