Don't forget to take into account not only the pipe weight but if making a platform on top the board weight as well.
Also Bill's numbers are accurate using inside diameter. Just an idea/tip though, if you require more displacement just use two T unions with a pipe down the center. Looks like every ~5' of pipe gives roughly 60 pounds of lift for 6".
Be careful if you try using SDR 35(6.275 O.D.)green sewer drain pipe it is not very compatible with Schedule 40 fittings designed for white pipe(6.625" O.D.) which is what most box stores stock. I feel it will cause headache and heartache when trying to make the floats watertight.
Hope some of this helps or makes sense running on little sleep.
Get the specific tees and elbows for SDR 35. It needs the same glue and primer as Sch 40. Or, use DWV pipe, which is thin wall and will take standard Sch 40 fittings. I looked again at my float made from 3 inch. It is DWV. A pipe feeding into my pond is SDR 35, and it has hubs on the ends and did not need couplings. My mistake....
