Put your boat in the water and measure how much the water rises on the pilings?
Seriously - It might help to think about it this way. Displacement is how much "water" the boat needs to displace in pounds. If it didn't displace the weight it would obviously sink.
What you might be interested in is "cubic" volume.
1 Pound of water = 0.0160187 Cubic feet
So a 5,000 lb boat will displace at least 80 cubic feet of water. This would be Delmarrey's full cup. Add freeboard, cabin
space etc. Greater "volume" will equate to greater bouyancy. I light high volume boat will be like a "cork" on the water. A heavy low volume boat may end up with lot's of water on deck
as it crashes through the swell.
To get a very rough idea of cabin
volume you can measure the top of the cabin to the top of the keel
height) double the measurement, solve for the volume of a cylinder and divide by about half.