Calculating your boat's displacement related to ballast weight may surprise you in two respects. The first is that your boat's displacement is effected by the ballast weight on a poundforpound basis, since your boat's displacement is the weight of your boat, reckoned in terms of the weight of water it displaces to float. The second way it will surprise you is in how much weight you may have to add or remove just to lower or raise your boat in the water by one inch.
Things You'll Need
 Calculator
 Owner's manual or documentation

Calculate the number of cubic feet of water your vessel is currently displacing. Subtract the amount of current freeboard (how far your boat sticks out of the water, from the deck to the level of the water on the hull) from the molded depth (available in your boat owner's manual or your boat's documentation) of the vessel. Multiply the result by the length of the vessel at the waterline (LWL) and the beam (width) of your vessel. This is the number of cubic feet of water your vessel is currently displacing.
The formula for this calculation is (Df)LWL x B,
where
D = molded depth,
f = freeboard,
LWL = vessel length at waterline and
B = beam of the vessel. 
Divide the amount of ballast to be added or removed by 64.1. Seawater weighs 64.1 pounds per cubic foot. The weight of the ballast equals the weight of the water it causes the vessel to displace.
If you remove 64.1 pounds of ballast, then the vessel will displace 1 cubic foot of water less than it did before the 64.1 pounds of ballast was removed. If you add 64.1 pounds of ballast, your boat will displace 1 cubic foot more than it did before the ballast was added.

Insert your vessel's dimensions in the formula to make the desired calculation. If your vessel's LWL is 20 feet long, with a molded depth of 5 feet and 3 feetof freeboard and a beam of 8 feet, the formula becomes (53) x 20 x 8, or 2 x 20 x 8 = 320, and you learn that your vessel is displacing 320 cubic feet, or 20,512 pounds of seawater.

Calculate how much effect adding or removing 64.1 pounds of ballast has on your boat by dividing the weight of water your vessel is displacing by your draftwhich equals the difference between your molded depth and your freeboard.
If your molded depth is 5 feet and your freeboard 3 feet, your draft is 2 feet. If the weight of the water displaced (called deadweight tonnage of the vessel) is 20,512 pounds, then each foot of draft is displacing 10,256 pounds of seawater and each inch of draft (10,256 / 12) represents 855 pounds of your boat's deadweight tonnage, including the ballast.
Tips & Warnings
 If you use your boat in fresh water, then the weight of the water is 62.4 pounds per cubic foot.
References
 "Merchant Marine Officers' Handbook;" Edward A. Turpin, William Alvin MacEwen; 1989