How Many Floats Does a Floating Dock Need?

Ask five dock suppliers how many floats you need and you’ll get five different formulas. Some say use 70% of the rated buoyancy. Some say 50%. Some skip the floats entirely and tell you to multiply square footage by 25.

They’re not contradicting each other. They’re all working backward from the same thing, and once you know what it is, the math stops being confusing.

The number that actually matters is freeboard

Freeboard is how far your deck sits above the water. That’s the thing you notice every time you use the dock - whether it feels solid and dry, or whether waves wash over it when a boat goes past.

For residential docks, the industry target is 12 to 14 inches of freeboard under normal load. High enough to stay dry in chop, low enough to step onto from a boat without climbing.

Every flotation formula you’ll find is just a shortcut for hitting that number. Which is why they look different but produce similar answers.

How freeboard actually works

Your deck height above water is two things added together:

The part of the float above the waterline, plus the thickness of your frame and decking.

A 2x6 frame is 5½″ and 5/4 decking adds about 1″, so the structure alone gives you roughly 6½″ before the float contributes anything. To reach 13″ of freeboard, you need about 6½″ of float riding above the surface.

On a 12″ float, that means about half of it sits underwater - which is where the “50% of rated” rule of thumb comes from. On a 20″ float, the same 6½″ above water means 13½″ submerged, or about 68% - hence the “70%” version. Same target, different geometry.

Working out your own number

Step 1: weigh the dock

Step 2: decide your live load

Step 3: check the freeboard, not the total

The shortcut

If you’d rather skip the arithmetic, the rule of thumb most suppliers use is 25 to 28 pounds of rated buoyancy per square foot of dock - the higher figure for heavier framing. Multiply, divide by your float’s rating, round up.

A worked example

A 4′ x 16′ dock, 2x6 frame, 5/4 decking, five 24″ x 48″ x 12″ floats rated 454 lbs each.

Empty, the dock uses about 25% of its flotation, putting 3″ of float underwater and leaving 15½″ of freeboard. Now load it up:

Sanity-check that against the shortcut: 64 sq ft × 25 = 1,600 lb of buoyancy required, which is 3.5 floats. Five gives you meaningful headroom over the minimum, which is why it’s the standard spec rather than four.

Notice too that the dock doesn’t sink at 5 adults - it just rides lower. Freeboard degrades gradually, which is what makes it a useful design target rather than a pass/fail limit.

Deeper floats do more work

Here’s something worth knowing before you order. Cost per pound of buoyancy at a typical 50% submergence, across the sizes we stock:

The 12″ series costs 30 to 70 percent more per pound of capacity than the deeper ones. If you want more freeboard or more capacity, going deeper is cheaper than going wider - and it means fewer floats to mount.

The constraint is water depth. A 24″ float needs enough water under it at your lowest seasonal level, plus clearance so it isn’t resting on the bottom in a drought year. On shallow shorelines the 12″ series isn’t a compromise - it’s the only thing that works.

Where the floats go

Placement matters as much as count.

Push the outer floats toward the ends. A dock with its flotation bunched in the middle see-saws when you walk to one end. Outer floats belong near the corners of the frame.

Keep it symmetrical across the width. Uneven side-to-side placement makes the dock list permanently, and it gets worse as the dock ages.

Bolt to the frame, not the decking. Flotation loads have to go into the structural members. PERMAFLOAT drums have cast-in mounting flanges for exactly this, and they want galvanized carriage bolts through the stringers with washer plates to spread the load. A bolt head pulling through wet pine is a slow leak of buoyancy you won’t notice until the dock lists.

Make sure the frame can take it. Flotation concentrates load at the mounting points, which is why cross joists need proper stringer angles rather than end-nailing. A frame that flexes works its float mounts loose over a season.

Compensate for anything heavy on one side. A gangway, a ladder, a bench at one end — add flotation under it rather than assuming the rest of the dock will carry it.

Replacing floats on an existing dock

Plenty of people arrive at this question the other way round — the dock already exists, and it’s sitting lower every year. A few things worth knowing before you order replacements.

Don’t assume like-for-like. If the dock rode low even when the floats were new, it was under-floated from the start. Replacement is your one cheap chance to fix that, since the labour is identical either way. Run the freeboard math above before you order.

Check the mounting geometry. Older drums used different flange patterns and spacing. Measure your existing mounting points before ordering, or plan on adding blocking to the frame.

Inspect the frame while it’s exposed. With the dock flipped or the floats off, you have access you won’t have again for years. Failed brackets, rusted-through angles, and loosened bolts are all far easier to replace now — and galvanized brackets and angles cost a fraction of what a second haul-out costs in effort.

Foam-filled beats air-filled. If your old floats were hollow and one of them failed, that’s the failure mode you’re replacing. Foam-filled drums don’t sink when punctured - they lose a little buoyancy and keep working.

Common mistakes

Designing to the rated number. That’s the fully-submerged figure. A dock at rated capacity is a dock at the waterline.

Forgetting the dock’s own weight. On a 4′ x 16′ it’s around 575 lb before anyone steps aboard - often the largest single load.

Sizing for the quiet afternoon. Docks disappoint on the busy day, not the average one.

Not accounting for what accumulates. Snow in northern climates. Lumber that gains weight over years in the water. The gear that gradually moves in and never leaves.

When in doubt, add a float

The failure is asymmetric. An extra float costs a few hundred dollars once. A dock that rides too low is something you notice every single time you use it, and retrofitting flotation to a finished dock means getting back under it in the water.

Building from scratch rather than adding flotation to an existing frame? Our complete dock kits ship with hardware and floats already matched to each size, and our guide to what a floating dock costs covers the full budget including lumber and freight.