Generally speaking, a manhole cover weights 90 to 150 pounds, according to Government Security News. A 32-inch cast-iron manhole cover can weigh up to 250 pounds at 450 pounds per cubic foot.
In the past, manhole covers were made of materials like cast iron to bear the weight of transport vehicles like buses and semi-trucks. Oil corporations started seeking for alternatives to use on their gas station storage tanks in the 1980s since these bulky coverings can result in serious physical harm to personnel.
Composite manhole covers, which have been demonstrated to offer a lighter and safer alternative to cast iron while still holding equivalent weight, were created in response by companies like Fibrelite.