The quantity contained in a sack WordNet 3.0 & Open English WordNet

The amount a sack holds; also, an archaic or historical measure of varying capacity, depending on commodity type and according to local usage; an old English measure of weight, usually of wool, equal to 13 stone (182 pounds), or in other sources, 26 stone (364 pounds). Wiktionary

The amount a sack will contain. Wiktionary