Displacement. One gallon of water weighs 8 pounds. For every gallon of water your boat displaces, that is 8 pounds. One gallon = 231 cubic inches. Shape is irrelevant.

Put another way, if your empty boat weighs, say, 40 pounds, it will displace 5 gallons of water. For every 8 pounds of paddler & cargo you add, another gallon (231 cu in) will be displaced.

If you want to retain, say, 6 " of freeboard, draw a line around your boat at that level, and calculate volume from there on down. A boat with flat bottom and straight sides would be easy.

Another way, is to draw the line around the boat, place the boat in water that is calm and fairly shallow, and start pouring in 5 gallon pails of water. When the boat sinks to the line, multiply the number of pails of water times forty pounds. That is your capacity to 6" of freeboard. Make sure the boat is not resting on the bottom. Probably want another person there to help stabilize the boat.

When done, grab one gunnel and roll the boat over to dump most of the water. When it is upside down and as empty as you can get it, snap roll it right side up again. This will capture the least amount of water. If the two of you can lift the empty boat, lift it up and carry it to shore.