How do I calculate displacement from weight?

250 lbs would displace 4.0 cu ft of fresh water and 3.9 cu ft of salt water. Divide the dry weight by 62.4 or 64 lbs respectively. To establish a waterline...that's another kettle of fish. Just where the weight is placed will affect the trim of the vessel but the larger the vessel the less affect the placement of moderate weights will have. 200 lbs in a 50 lb boat will have an enormous effect for even a small movement of the center of mass, whereas 200 lbs in a 5000lb boat won't do all that much at all.

Steve