Government Security News reports that the average manhole cover weighs between 90 and 150 pounds. At 450 pounds per cubic foot, a 32-inch manhole cover made of cast iron may weigh up to 250 pounds.
Originally, manhole covers were constructed from materials such as cast-iron to bear the weight of vehicles such as buses and semi-trucks. In the 1980s, oil corporations began searching for alternatives to use on their gas station storage tanks, as these hefty coverings can cause severe physical harm to personnel. Companies such as Fibrelite responded by manufacturing composite manhole covers, which have been demonstrated to be a safer and lighter alternative to cast iron while maintaining the same load capacity.