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Dock flotation calculator
One 8′ × 20′ aluminum-frame floating section on the residential basis needs 4,480 lb of catalog buoyancy — 10 drums of 500 lb. This calculator shows all three criteria the dock engine sizes against, which one governed, and the source behind every number — including the ones that have none.
Your dock
Floats on drums — the only type with flotation to size.
Sections run end to end. Each can be up to 20 ft wide and 60 ft long.
Frame and decking set the dead load — the one input here with no published source.
20 psf deck live load and a 28 psf flotation floor. Live load: Ameren Lake of the Ozarks Permit Requirements, Appendix B §7.7.1 (permit-mandated on that lake) (Tier 1 — legal force).
Sized in 24"×48"×16" foam-filled float drums, rated 500 lb at full submersion.
Site conditions are held neutral — 4 ft of water at seasonal low and high, a sand bottom, sheltered exposure, no jurisdiction recorded — because flotation never reads them. They drive the dock studio's site checks, not this number.
Required flotation · residential basis
10 float drums
4,480 lb of catalog buoyancy — 28 psf over 160 sq ft, set by published total-flotation floor.
Required buoyancy
4,480 lb
160 sq ft × 28 psf, counted at full-submersion catalog ratings.
Tier 3 — documented practice Dock Builders Supply — the floor of its published residential total-flotation band
Float drums
10
24"×48"×16" foam-filled float drums rated 500 lb each — whole drums, an even count per section so they pair across the width.
Tier 3 — documented practice Dock Boxes Unlimited H2448-16 catalog row, held at the round conservative figure; Formex: published buoyancy is for total submersion
Live load
20 psf
The residential basis, carried at full value into the reserve-buoyancy case.
Tier 1 — legal force Ameren Lake of the Ozarks Permit Requirements, Appendix B §7.7.1 (permit-mandated on that lake)
Dead load
720 lb
3 psf frame + 1.5 psf decking = 4.5 psf.
Gap — no published source No published source — documented practice gives only aggregate dead-load benchmarks, so the per-material frame and decking splits are estimating placeholders pending marine-engineer review
The three criteria — the worst one governs
Dead load at design submergence
10 psf
The dock at rest rides no deeper than 45% of the drum’s height.
4.5 psf dead ÷ 0.45 = 10 psf
Tier 3 — documented practice Dock Builders Supply published flotation method — the dock at rest rides no deeper than the design submergence
Reserve buoyancy at full live load
28 psf
At dead load plus the full 20 psf live load, at least 2 in of the 16 in drum stays above water.
(4.5 dead + 20 live) ÷ 0.875 = 28 psf
Tier 2 — published guideline UFC 4-152-07, Design: Small Craft Berthing Facilities §6-3.3.2
Published total-flotation floor
28 psfgoverns
The residential basis never drops below its published total, even when the loads ask for less.
28 psf floor
Tier 3 — documented practice Dock Builders Supply — the floor of its published residential total-flotation band
What the count delivers: 10 × 500 lb = 5,000 lb, or 31.3 psf of deck. At rest the drums ride 14% submerged (the limit is 45%); at dead plus full live load 3.5 in of drum stays above water (the minimum is 2 in).
Catalog buoyancy is a full-submersion figure, so the load is never compared with the rating directly. Deck freeboard, anchoring, wind and wave are not checked here — see every source and gap.
How the number is sized
A float drum's catalog rating is measured fully underwater, so a flotation number is only honest if it says how deep the drums ride. The dock engine sizes against three criteria and takes the worst. For the 8′ × 20′ section the calculator opens with:
Dead load at design submergence
10 psfThe dock at rest rides no deeper than 45% of the drum’s height.
4.5 psf dead ÷ 0.45 = 10 psf
Tier 3 — documented practice Dock Builders Supply published flotation method — the dock at rest rides no deeper than the design submergence
Reserve buoyancy at full live load
28 psfAt dead load plus the full 20 psf live load, at least 2 in of the 16 in drum stays above water.
(4.5 dead + 20 live) ÷ 0.875 = 28 psf
Tier 2 — published guideline UFC 4-152-07, Design: Small Craft Berthing Facilities §6-3.3.2
Published total-flotation floor
28 psf · governsThe residential basis never drops below its published total, even when the loads ask for less.
28 psf floor
Tier 3 — documented practice Dock Builders Supply — the floor of its published residential total-flotation band
160 sq ft at 28 psf is 4,480 lb, which takes 10 drums once the count is rounded to whole drums and paired across the width. Make the same section pressure-treated wood and the dead load climbs to 8.5 psf, the reserve buoyancy at full live load rule takes over at 32.6 psf, and the count becomes 12.
What it refuses to estimate
Two docks get no number, on purpose. A piling or pipe dock stands on the bottom — nothing on it floats, so there is nothing to size. And round or pipe pontoons load non-linearly, which the rectangular-drum math cannot describe; rather than stretch it over them, the engine stops and says why:
Circular / pipe pontoons load NON-LINEARLY (Tier 2, CA DBW 2005 §C6.8.2: the lower half of the diameter gives disproportionally greater buoyant uplift and the upper half disproportionally less), while this engine sizes flotation with the linear rectangular-pontoon relationship of §C6.8.1 against a rectangular-drum catalog rating. UFC 4-152-07 §6-3.3.2 adds that where pontoon width varies with submergence the reserve freeboard must be INCREASED by an amount it does not quantify. No float count was produced — the engine will not extrapolate its rectangular model onto a shape it does not describe.
What to do: Size pipe/circular flotation from the manufacturer's own displacement curve or a marine engineer's calculation, or switch floatShape to rectangular drums to get a takeoff from this engine.
Common questions
How many float drums does an 8x20 floating dock need?
10 drums rated 500 lb each, for one 8′ × 20′ aluminum-frame, aluminum panel-deck section on the residential basis — 4,480 lb of catalog buoyancy, which is 160 sq ft at 28 psf, set by the published total-flotation floor. The count rounds up to whole drums and then to an even number per section, so the drums pair across the width. The same section in pressure-treated wood needs 12 drums, because its heavier dead load hands the answer to the reserve buoyancy at full live load rule.
How much flotation does a floating dock need per square foot?
The worst of three published criteria, not one rule of thumb. Dead load alone may sink the drums no deeper than 45% of their height (Tier 3 — documented practice: Dock Builders Supply published flotation method — the dock at rest rides no deeper than the design submergence). At dead load plus full live load, the reserve-buoyancy rule keeps at least 2 in of a 16 in drum above water (Tier 2 — published guideline: UFC 4-152-07, Design: Small Craft Berthing Facilities §6-3.3.2). And there is a published floor — 28 psf residential, 40 psf marina (Tier 3 — documented practice). A light aluminum dock sits at 28 psf; a pressure-treated wood one asks 32.6 psf.
Why not divide the dock’s weight by the drum’s buoyancy rating?
Because catalog buoyancy is a full-submersion figure: a 500 lb drum delivers 500 lb only when it is completely underwater, with the dock awash. Formex prints that caveat on its own specification page. So the rating is never compared with the load directly — the dead load is held to 45% submergence and the full live load to the reserve line. On the 8′ × 20′ section above, the 10 counted drums leave the dock 14% submerged at rest, with 3.5 in of drum still above water at full load.
Residential or marina load basis — which should I use?
Whichever your dock actually is, and never one for the other. Residential designs to 20 psf of deck live load (Ameren Lake of the Ozarks Permit Requirements, Appendix B §7.7.1 (permit-mandated on that lake); Tier 1 — legal force) with a 28 psf flotation floor. Marina designs to 25 psf (CA DBW, Layout and Design Guidelines for Marina Berthing Facilities (2005) §C2.2; Tier 2 — published guideline) with a 40 psf floor. The same 8′ × 20′ section needs 10 drums residential and 14 marina.
Can it size round or pipe pontoons?
No, and it says so instead of estimating. Every number here describes a rectangular drum, whose waterline area stays the same as it sinks; a round pontoon's does not, so it loads non-linearly. Choose round pontoons and the engine returns no float count and raises DOCK-BP-FLOAT-SHAPE, with this remedy: Size pipe/circular flotation from the manufacturer's own displacement curve or a marine engineer's calculation, or switch floatShape to rectangular drums to get a takeoff from this engine.
Is there a code for dock flotation?
There is no national dock code. Nothing national sets how much flotation a dock carries. What exists is a ladder: your waterway authority — typically the U.S. Army Corps of Engineers district, then your state waterway agency, then local shoreline zoning — decides what you may build; published design guidelines sit below that; documented manufacturer practice below that. This calculator names the rung of every number it uses, and the dock source register publishes the documents, their checksums and the gaps.
What does this calculator not check?
The frame and decking dead loads have no published source — they are estimating placeholders, labelled that way, pending marine-engineer review. Deck freeboard is not checked against the published bands, and anchoring, wind, wave and ice are site questions this tool does not ask: the site is held neutral (4 ft of water at seasonal low and high, a sand bottom, sheltered exposure, no jurisdiction recorded) because flotation never reads it. A dock carrying a crowd, a vehicle or a boat lift belongs with a marine engineer.
Now take off the whole dock
The dock studio runs this same engine across the whole job — floats, frame, decking, cleats, anchoring and gangway — with a 3D waterfront preview, site checks that name their authority, and a branded proposal. A 14-day free trial shows the priced totals and a preview of the takeoff; the full line-item sheet and its exports come with a plan.
No card required.
Source & limits
There is no national dock code.
Every number above names its rung on the authority ladder — rules with legal force, published design guidelines, documented manufacturer practice — or says plainly that no published source exists. The documents, their checksums and every gap are in the dock source register.
Estimate only — not an engineered design. THERE IS NO NATIONAL DOCK CODE: these quantities are sized to documented industry practice and published design guidance — flotation at a design submergence with reserve freeboard, never at catalog full-submersion ratings — and not to a prescriptive code table. Each line states the basis it was sized on. Your waterway authority governs: typically the U.S. Army Corps of Engineers district, then your state waterway agency, then local shoreline zoning, each setting its own dimensional limits. Verify everything with your permitting authority and a licensed marine contractor or engineer before ordering or building.