Your next step is to calculate how many floats you will need to float the live and dead loads you've just calculated. Start by consulting Table 1 below, which shows how much weight each size float will support at four different depths of submergence. Decide which float size you wish to use and how deeply you want to submerge it. Then select from Table 1 the accompanying buoyant force for that float at that submergence and divide your calculated dead load by that buoyant force. Under normal everyday conditions, the floats should never be submerged more than 50%.

For example, assume a small 10'x12' swim dock with a calculated dead load of 1440 pounds. Table 1 shows that a 24" x 48" x 16" float will support 239.2 pounds when 40% of it is submerged. So, six of these floats will support the raft (1440/239.2 equals 6), which will leave 60% of each float above the waterline (a freeboard of 9.6 inches). The available live load which can now be supported on this swim raft is 2184 pounds. Here's the supporting calculation. Table 1 shows that it takes 3588 pounds to submerge these six floats to 100% (6 x 598 equals 3588). So then 3588 pounds of buoyant force minus the 1440 pound dead load of the dock leaves 2148 pounds of available live load. It's important to remember that the live load will NEVER be distributed equally, so always add extra flotation if you feel you may come anywhere close to maximizing out the available live load. 

Your final step is to determine how many square inches of the structure's cross-members you should place in contact with the floats to transfer the structure's weight to the floats. To determine the size of this float contact area in square inches, multiply the dead load of the structure by the appropriate Design Factor in Table 2 (based on the expected wave action). Since this is the contact area for the entire structure, and since you want to determine the contact area for each cross-member which will be bearing on the floats, divide your answer by twice the number of floats you'll be using (assuming a minimum of two cross-members per float).

For example, if the 10'x12' swim dock is located on an inland lake, multiply its dead weight (1440) by the Design Factor for inland lakes, found in Table 2 (0.32). Thus, 1440 x 0.32 equals 461 square inches of drum surface, which equates to 77 square inches of contact area for each of the six supporting floats (461/6 equals 77). Since each float must have contact with at least two cross-members of the structure, each cross-member should have at least 38.5 square inches in contact with the float  (77/2 equals 38.5).

¹Freeboard is the vertical distance between the water level and the top of the float.

²The buoyant force is the load that can be carried by one billet at the stated percent submergence.

Table 2. Design Factors