How do you calculate fiberglass boat weight?

It would be much easier to determine the weight of the water it displaces, if you are unable to place the boat on a scale. Place the boat in the water, mark the water line on the hull. Remove the boat from the water and measure the dimensions or volume of the hull below the water line. then multiply this dimension by the density of water, 62.4 lbs/ cu. ft. (or 1 Kg/dM). The result will be the empty weight of your boat.

for instance a flat bottomed boat, 18 ft long, with a 6 foot beam, drafting 4 inches will displace approximately 36 cu. ft. of water this corresponds to a weight of 2220 lbs.