Stone
The weights of items in Ultima Online are measured in stones.
Normal containers that have not been locked down in a house have a capacity of 400 stones. A character's Backpack may hold 550 stones. Humans may carry 60 more stones then Elves.
A stone is 14 pounds (~6.35029318 kilograms). A character can potentially walk with over 600 stones (when using items with strength enhancing properties along with magical buffs such as the Bless spell), which translates to 8,400 pounds (~3.8 metric tons).
On the other hand, the 1600 stones a pack animal can carry translates to 22,400 pounds, or just over 10 metric tons.
Characters weigh 13 stones, which is 182 pounds, or 82 kilos.