Pounds are force, just like newtons. Weight is usually the force due to gravity. Since the acceleration due to gravity is approximately a constant at the earth''s surface, it is possible to convert from pounds to kilograms by calculating the mass needed (in kilograms) to produce the object''s weight at the surface of the earth (in pounds).

Your formula for displacement force, density * volume, is incorrect because that formula only gives the mass of the displaced substance. Assuming that acceleration due to gravity is a constant (which is acceptable at the earth''s surface) allows you to use force per volume * volume, which is what you were actually doing with 62.4 lbs per square foot times the volume.

The reason for the force is this: Pascal''s principle states that in an incompressible and static fluid, each point in the fluid exerts its pressure in all directions equally. The pressure at a point in the fluid increases with depth, because the deeper you go, the more fluid there is above pushing downward. Making a few approximations, we find that the fluid at the top of the object is exerting a downward force and the fluid at the bottom of the object is exerting an upward force on it (there are also forces pushing at the sides of the object, but these all cancel out, since for all the surface area on one side of the object there must be an equal amount of surface area on the other side). The force at the bottom is greater than the force pushing down from the top because the fluid at the bottom is deeper and is under higher pressure. The net force is approximately the mass of the displaced fluid * the acceleration due to gravity (which we are assuming to be a constant, about 9.8 m/s^2 near the earth''s surface).

Perhaps it is easiest to imagine a rectangular object in the water. The top and bottom of the object have a surface area of A, and the distance between the top and bottom sides is D, and the water has density d, and the top of the object is at depth x (too many things start with d...), and acceleration is g. The volume of water above the object pushing down is A * x, so its mass is A*x*d, and the force it is exerting is A*x*d*g. The volume of the water which is above the water which is pushing up on the bottom of the object is A*(x+D), and we get that the total force pushing up is then A*(x+D)*d*g.

From here we find the total upward force is A*(x+D)*d*g - A*x*d*g. Simplifying leads to a net force of A*D*d*g. A*D is the volume of the object, V, so we end up with V*d*g, volume times density times acceleration due to gravity. Now I''m sure there''s some advanced way to prove that this works for arbitrarily shaped bodies, with all the forces cancelling out except for the upward force of V*d*g, but I think this is enough, and you can sort of imagine how the forces cancel out for other shapes.

pounds can be force OR mass. Just make sure you make the distinction in your units. Use lbf or lbm. They are equivalent at sea level on earth.

To convert you need a gravitational constant and acceleration due to gravity. Gc = 32.2 lbm*ft / lbf*s*s

So,
1lbm / 32.2 lbm*ft/lbf*s*s * 32.2ft/s*s = 1lbf
(the units cancel, trust me..its hard to do this on something other than paper)