A king-size waterbed mattress can hold approximately 235 gallons of water. This size of waterbed, depending on the manufacturer, could weigh up to 2,000 pounds.
Despite the massive weight of a king-size waterbed, most structures built according to modern building codes should be able to support the weight of the bed. For comparison, a queen-size waterbed contains about 196 gallons of water. One additional factor that can affect the weight of a waterbed is whether it has a hardside or softside mattress. Hardside beds have hard fiber within the material of the frame, making them more robust and heavier, while softside beds have a foam rubber frame.