The two most common methods of constructing floating docks use either empty 55 gallon metal storage barrels or large sections of styrofoam. Styrofoam is more versatile and therefore the preferred choice for building custom dock floats. Many vendors can cut custom marine-grade styrofoam to meet the specific needs of any dock project, but standard stock sizes are sufficient for most projects.
Estimate the maximum load that the dock will have to support. This can be done by multiplying the number of people expected to be on the dock at any one time by the average weight of the people. The current average weight for an adult male in America is approximately 86.2 Kilogram.
Divide the maximum load for the dock by 55. Standard styrofoam has a buoyancy of 24.9 Kilogram per cubic foot, so dividing the maximum load by 55 will result in the cubic feet of styrofoam required to adequately float the dock.
Determine if standard available sizes of styrofoam billets can be used for your existing dock. A common standard size for dock floats is the 10-inch by 24-inch by 96-inch billet. This is a 13.3 cubic foot piece of styrofoam that provides approximately 331 Kilogram of buoyancy.
Use a large handsaw to cut the styrofoam billets in instances where the standard sizes will not fit beneath a dock.
Detailed plans for building swimming rafts and floating docks are available from the manufacturer of styrofoam billets listed in the Resource link.