Flotation Calculation - Dock Floats

Calculating the amount of flotation can sometimes be difficult, but here are a few simple ways to calculate how much flotation you need for most non-covered residential wooden docks.
(If you have a more complicated dock situation with unusual loads, please scroll down.)

Take the square footage of your dock and multiply it by the multiplier number in the table below to find out how much total buoyancy you need for your dock. The total buoyancy for each of our dock floats is listed on our dock floats page. Divide the total required buoyancy by the buoyancy per float to determine the number of floats you will need.

Example #1

A 12' by 12' floating dock constructed with 2" x 8" framing and 5/4" x 6" pressure treated decking would be calculated as follows:

12 x 12 = 144 square feet x 28 = 4032 lbs of buoyancy required. You will typically want to have floats placed in all corners and no more than 8' apart. Whether you use several smaller floats or fewer larger floats is totally up to you as long as you achieve at least the total recommended buoyancy.

With this in mind, take a look at our extensive selection of floats at Our Dock Flotation Page . In this example, you can use 6 of the DF-364812 (36" x 48" x 12" floats) which yield a total buoyancy of 4146 lbs.

Example #2

A 12' by 12' floating dock constructed with 2" x 8" framing and 5/4" x 6" composite decking would be calculated as follows:

12 x 12 = 144 square feet x 31 = 4464 lbs of buoyancy required. In this example, you can use 4 of the DF-486012 (48" x 60" x 12" floats with 1190 lbs of buoyancy each) which yield a total buoyancy of 4760 lbs.

Example #3

An 8' by 16' floating dock constructed with 2" x 8" framing and 2" x 6" pressure treated decking would be calculated as follows:

8 x 16 = 128 square feet x 31 = 3968 lbs of buoyancy required. In this example, you can use 6 of the DF-364812 (36" x 48" x 12" floats with 691 lbs of buoyancy each) which yield a total buoyancy of 4146 lbs.

Example #4

A 10' by 20' floating dock constructed with 2" x 8" framing and 5/4" x 6" composite decking would be calculated as follows:

10 x 20 = 200 square feet x 31 = 6200 lbs of buoyancy required. In this example, you can use 8 of the DF-364816 (36" x 48" x 16" floats with 905 lbs of buoyancy each) which yield a total buoyancy of 7240 lbs, or you can use 6 of the DF-4512 (48" x 60" x 12" floats with 1190 lbs of buoyancy each) which yield a total buoyancy of 7140 lbs.

For More Complicated Dock Systems with Unusual Loads

1. Determining Your Live and Dead Loads

Your first step is to determine the live and dead loads of your floating structure.

The dead load is the weight of the framing, decking, connections, flotation units, and all permanently-attached equipment, such as pipes, pumps, utilities, benches, etc. As a general rule of thumb, the dead weight on most residential docks that are constructed using lumber is typically between 10 and 15 lbs/ft² of structure.

The live load is essentially the weight of the people and gear that will be placed on the floating structure. It is recommended that the structure be designed for approximately 40% submergence, so that the remaining 60% can be used to support the live load.

2. Determining the Quantity & Size of Floats Needed

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' x 12' 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.

3. Determining Bearing Area Needed

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' x 12' 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).

¹ Freeboard is the vertical distance between the water level and the top of the float.
² The buoyant force is the load that can be carried by one billet at the stated percent submergence.
³ Dock floats are not designed to be fully submerged. Values indicate displacement-related total buoyancies for reference only.