How To Calculate Flotation

  • Weight of wood and hardware = 5-7lbs per square foot (based on 2 x 6 cedar)
  • Weight of people and furniture = variable
  • Dock with floats should average sinking 20-30% into the water leaving 70-75% remaining floatation for “variable” before dock submerges
  • Dock square footage multiplied by 25 divided by floatation capacity = amount of floats required
  • Dock weight divided by floatation capacity = % dock (float) will sink with no people on it yet
  • Dock weight divided by flotation required x float height = # inches dock (float) will sink with no people on it yet
Example 1
Dock 4′ x 10′ sq. ft. @ 6 lbs per square foot = 240 lbs
Variable – 4 people @150 lbs = 600 lbs
FLOATATION REQUIRED = 840 lbs
Thus 40 sq. ft. x 25 lbs per square foot divided by the capacity of the dock float (Howell 400) = 3
Three Howell 400 floats @ 400 lbs each = 1200 lbs floatation
Float will sink 20% or 3.42″
This design would be useful as a walkway for docking only – float height 12″ and should be anchored for lateral stability.
 
Example 2
Dock 6′ x 10′ sq. ft. = 60 sq. ft. @ 6 lbs per sq. ft = 360 lbs
Variable 5 people @ 150 lbs = 750 lbs
FLOATATION REQUIRED = 1110 lbs
Thus 60 sq. ft. x 25 lbs per square foot divided by the capacity of the dock float (Howell 550) = 3
Three Howell 550 floats @ 550 lbs each = 1650 lbs floatation
Float will sink 22% or 5.2″
This design is a stable configuration useful for medium size docking and joining sections for combo walkway dock and can be built in longer lengths.
Example 3
Dock 8′ x 10′ = 80 sq. ft. @ 6 lbs per sq. ft = 480 lbs
Varibable – 5 people @ 150 lbs = 750 lbs
FLOATATION REQUIRED = 1230 lbs
Thus 80 sq. ft. x 25 lbs per square foot divided by the capacity of the dock float (Howell 400) = 5
Five Howell 400 floats @ 400 lbs each = 2000 lbs floatation
Float will sink 24% or 4.7″
This design would be useful for dock/floating dock accommodating more people and accessories.
4 x Howell @ 550 lbs = 2200 lbs floatation could be used to accommodate additional weight and raise dock for easier boat access, float height 16″
Example 4
Dock 10′ x 12′ = 120 sq. ft. @ 6 lbs per sq. ft = 720 lbs
Variable – 7 people @ 150 lbs = 1050 lbs
FLOATATION REQUIRED = 1770 lbs
Thus 120 sq. ft. x 25 lbs per square foot divided by the capacity of the dock float (Howell 550) = 5
Five Howell 550 floats @ 550lbs each = 2750 lbs floatation
Float will sink 26% or 6.5″
This design would be useful for dock/floating patio application float height 16″
Adding 1 more 550 float = 3300 lbs allowing for more people, chairs, accessories chairs, etc. and keeping dock high so boat wake, wave action does not splash over top.
Float will sink approx. 22% or 5.2″