According to Government Security News, the average weight of a manhole cover is between 90 and 150 pounds. A 32-inch cast-iron manhole cover can weigh as much as 250 pounds due to its density of 450 pounds per cubic foot.
Cast-iron was once commonly used in the construction of manhole covers so that they could bear the weight of heavy vehicles like buses and semis. Oil companies started seeking for alternatives to using these heavy covers on their gas station storage tanks in the 1980s because of the danger they posed to workers. As a result, manufacturers like Fibrelite developed composite manhole covers, which have proven to be a lighter and safer alternative to cast iron.