Floating Dock Design Calculator
Calculate buoyancy, draft, freeboard, reserve buoyancy, and metacentric height (GM) for rectangular floating docks, pontoons, and barges.
About this calculator
This calculator treats a floating dock as a simple rectangular pontoon and applies Archimedes' principle: at equilibrium, the weight of water displaced equals the total mass (dead weight plus live load), so draft is found by dividing displaced volume by waterplane area (length times beam). Freeboard is just the pontoon's total depth minus that draft, and reserve buoyancy expresses, as a percentage, how much additional load the structure could take before the deck itself goes under — a useful margin-of-safety number distinct from the load rating alone. Stability is checked separately via the metacentric height, GM = KB + BM − KG: KB (center of buoyancy above keel) is half the draft, BM (metacentric radius) comes from the waterplane's second moment of area for a rectangle (L×B³/12) divided by displaced volume, and KG is the vertical center of gravity you supply.
A positive GM means the pontoon rights itself when tilted; naval architecture practice generally wants GM comfortably above 1.0 m for a dock carrying pedestrian or vehicle loads. If the calculated draft would exceed the pontoon's total depth, the calculator flags the structure as overloaded and caps the draft used in the stability math at the physical depth — a sign the design needs more freeboard or buoyancy, not just a larger number to report. Keep in mind this model assumes a rectangular hull with uniform properties; real docks with notches, walkways, or non-rectangular footprints need a more detailed hydrostatic analysis.
Inputs
Results
Draft (m)
0.57
Freeboard (m)
1.43
Metacentric height GM (m)
4.56
How to Use This Calculator
- Enter pontoon dimensions: length (m), beam (m), and depth (m).
- Enter dead weight (kg) and live load (kg) for the pontoon.
- Read draft (m), freeboard (m), and displacement (tonnes).
- Check GM (metacentric height, m) for stability — positive GM indicates stable upright equilibrium.
- Verify reserve buoyancy (%) and load capacity to ensure safe operational margins.
How the result changes with Pontoon length (m)
| Pontoon length (m) | Draft (m) | Freeboard (m) | Metacentric height GM (m) |
|---|---|---|---|
| 10 | 1.14 | 0.86 | 2.21 |
| 15 | 0.76 | 1.24 | 3.33 |
| 30 | 0.38 | 1.62 | 7.1 |
| 50 | 0.23 | 1.77 | 12.29 |
What each input means
- Pontoon length (m)
- Overall length of the pontoon or floating dock.
- Pontoon beam / width (m)
- Overall beam (width) of the pontoon.
- Pontoon depth (m)
- Total depth (height) of the pontoon hull from keel to deck.
- Dead weight (kg)
- Self-weight of the pontoon structure (hull, frames, deck).
- Live load (kg)
- Maximum variable load (vehicles, people, equipment).
- KG — center of gravity above keel (m)
- Vertical distance from the keel to the combined center of gravity.
- Water density (kg/m³)
- Density of the water. Seawater ~1025, fresh ~1000 kg/m³.
What each result means
- Draft (m)
- Depth of submergence below the waterline at equilibrium.
- Freeboard (m)
- Height of the deck above the waterline. Must remain positive.
- Displaced volume (m³)
- Volume of water displaced at equilibrium.
- Reserve buoyancy (%)
- Percentage of total hull volume not submerged — margin before swamping.
- Metacentric height GM (m)
- Transverse stability indicator. GM > 0 is stable; typically > 1.0 m for docks.
- Displacement (tonnes)
- Total displacement mass.
- Tonnes per cm immersion
- Additional mass required to increase draft by 1 cm.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPontoon length (m) = 20, Pontoon beam / width (m) = 6, Pontoon depth (m) = 2, Dead weight (kg) = 50000 = 7 input(s) provided
- Calculate DraftDraft = displacedVolume / waterplaneArea0.569 = 0.569
- Calculate FreeboardFreeboard = depth - effectiveDraft1.431 = 1.431
- Calculate Metacentric height GMMetacentric height GM = KB + BM - KG4.556 = 4.556
- Calculate Displaced volumeDisplaced volume = totalMass / rhoWater68.29 = 68.29
- Calculate Reserve buoyancyReserve buoyancy = ((maxDisplacement - totalMass) / maxDisplacement) * 10071.5 = 71.5
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What does it mean if the calculator flags the pontoon as overloaded?
It means the total mass (dead weight plus live load) divided by waterplane area produces a draft deeper than the pontoon's actual hull depth — physically, the deck would be underwater and the structure would be swamped rather than floating at that load. When this happens the calculator caps the draft used in the freeboard and stability math at the physical hull depth, since draft can't exceed depth in reality, and the result should be read as a signal to reduce load or increase pontoon size, not as a usable design point.
Why does GM use KB plus BM minus KG instead of just the center of gravity?
Metacentric height measures the net righting effect when the pontoon tilts, which depends on how the center of buoyancy shifts relative to the center of gravity as the hull heels. KB (half the draft) locates where buoyancy currently acts, BM (the waterplane's second moment of area divided by displaced volume) captures how much the buoyancy center shifts sideways as the vessel tilts, and subtracting KG — the vertical center of gravity you supply — gives the actual margin between the righting and capsizing tendencies; a bigger BM relative to KG means a wider, shallower-draft pontoon is inherently more stable.
Why does reserve buoyancy matter separately from the load rating?
Reserve buoyancy is the percentage of the pontoon's total hull volume above the waterline at the current load — essentially how much more weight could be added before the deck submerges, expressed relative to full hull capacity rather than a fixed rated tonnage. It's a useful safety-margin check because two docks can carry the same rated load but have very different reserve buoyancy depending on their depth and waterplane area, and low reserve buoyancy means less warning before swamping if load is added unexpectedly.
Does this calculator work for docks that aren't rectangular pontoons?
Not accurately — the draft, freeboard, and especially the BM term in the stability calculation all assume a rectangular waterplane with the second moment of area formula L×B³/12. Docks with notches, cutouts, walkways, or irregular footprints have a different actual waterplane geometry and second moment of area, so their real stability characteristics will differ from what this simplified rectangular model reports; those designs need a proper hydrostatic analysis of the actual hull shape.
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