How To Calculate Flotation

To calculate floatation for a dock, start by estimating the weight of the wood and hardware, typically 5–7 lbs per square foot using 2x6 cedar, and add the variable weight of people and furniture. A properly designed dock should sink approximately 20–30% into the water, leaving 70–75% of floatation capacity available for variable loads before submersion. To determine the number of floats required, multiply the dock’s square footage by 25 and divide by the float's capacity.

To calculate how much the dock will initially sink without any added weight, divide the dock’s weight by the float’s capacity to get a percentage, then multiply this by the float height to get the number of inches it will sink.

For example, a 4′ x 10′ dock weighing 240 lbs with a 600 lb variable load requires 840 lbs of floatation; using three 400lb floats (400 lbs each) provides 1200 lbs of total floatation, causing the dock to sink approximately 3.42″ or 20%, suitable for a light walkway.

A 6′ x 10′ dock with five people needs 1110 lbs of floatation; three 550 floats offer 1650 lbs, causing a 22% sink, ideal for medium dock use.

An 8′ x 10′ dock with five people requires 1230 lbs, and five 400lb floats give 2000 lbs, resulting in 24% sink—suitable for larger docks. Alternatively, four 550lb floats would offer 2200 lbs, raising the dock for better boat access. 

A 10′ x 12′ dock with seven people needs 1770 lbs of floatation; five 550lb floats provide 2750 lbs, with a 26% sink (6.5″), ideal for floating patio setups. Adding an extra float increases capacity to 3300 lbs, reducing the sink to 5.2″ and improving performance in waves and wakes.