Marina Berth Size and Clearance Calculator
Vessel, berth, and margins
Berth fit result
Enter vessel, berth, and margin details to calculate.
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Calculation method
The calculator treats the boat and selected margins as a rectangular planning envelope. It compares each required dimension with the corresponding usable berth dimension.
Required length = LOA + bow margin + stern margin
Required width = beam + 2 × side margin
Required depth = loaded draft + UKC margin
Required overhead = air draft + overhead margin
A positive surplus means the selected margin is met on that axis. Zero meets it exactly; a negative value is the shortfall. Width assumes equal free space on both sides when centred.
Measure before comparing
- Confirm whether the marina counts bowsprits, anchors, swim platforms, davits, outboards, rub rails, and fenders.
- Use the berth's actual usable geometry after piles, cleats, ladders, neighbouring boats, and other obstructions.
- Match draft, depth, air draft, and overhead clearance to the same loading and water-level condition.
- Enter margins supplied by the marina or derived from qualified local guidance—not assumed universal values.
- Review every axis and confirm the result directly with the marina before arrival.
Worked example
A 36 ft boat with 12 ft beam and 5 ft loaded draft is compared with a 42 ft by 16 ft berth having 7 ft of controlling depth. With 1.5 ft at each end, 0.75 ft on each side, and 0.5 ft under the keel:
Required envelope = 39.0 ft long × 13.5 ft wide × 5.5 ft deep
Surplus = 3.0 ft length, 2.5 ft width, and 1.5 ft depth
The arithmetic passes all three selected margins, but the operator must still confirm actual geometry, measurement definitions, and operating conditions.
Marina berth FAQs
Should I enter registered length or length overall?
Use the full dimension the marina uses for allocation and billing. This is often length overall, including specified projections, but definitions vary. Ask whether anchors, bowsprits, pulpits, platforms, davits, tenders, and tilted or raised outboards count.
How is clear berth width different from nominal slip width?
Clear width is the narrowest usable space available to the vessel after fixed structures and relevant obstructions. A nominal class or centreline spacing may not equal the opening between piles, pontoons, fenders, or adjacent vessels.
What side clearance should I use?
Use the marina's minimum or an appropriate vessel- and site-specific value. The necessary gap varies with fenders, hull form, piling layout, wind, current, wakes, surge, neighbouring boats, and manoeuvring method.
What water depth should I use?
Use a reliable controlling depth for the berth and immediate approach at the relevant low-water condition and datum. Allow for loading, squat, heel, motion, uncertainty, sedimentation, and applicable minimum under-keel requirements outside this simple arithmetic.
When should I enter air draft and overhead clearance?
Enter both when a roof, cable, bridge, travel-lift structure, or other vertical limit affects access or occupation. Use the vessel's highest relevant fixed point and the lowest obstruction at the same water level; leave both blank when no overhead check is needed.
Can the boat fit if one result fails?
Not under the selected assumptions. Each axis is independent, so extra length cannot compensate for insufficient width, depth, or overhead clearance. Resolve the shortfall with the marina rather than relying on the overall label alone.
Does a pass guarantee safe docking?
No. The result is only a dimensional screen. It does not assess approach channel limits, turning room, propulsion, windage, weather, current, mooring loads, fendering, shore services, emergency access, or operator rules.
Are my boat dimensions uploaded or tracked?
No. Calculations run locally in the browser, and this tool does not send, store, or attach entered dimensions to analytics events.
Limits and safety disclaimer
- The calculator assumes a rectangular envelope and, for width, a centred vessel with equal clearance on each side.
- It does not model boat movement, berth taper, diagonal placement, pile spacing, line geometry, fender compression, overhang rules, or adjacent-vessel movement.
- Depth and overhead values are only comparable when referenced to a consistent water level; tides, river stages, loading, squat, heel, waves, and survey uncertainty can change clearance.
- Marinas may impose separate maximum length, beam, draft, displacement, hull-type, insurance, utility, and access limits regardless of calculated fit.
Safety disclaimer: This is an educational planning aid, not navigation, seamanship, mooring engineering, or marina approval. Obtain the controlling limits and measurement definitions from the marina or harbour authority and use qualified local advice where consequences are material.
Methodology and sources
Last reviewed: August 2, 2026. The arithmetic is explicit user-input geometry. Safety context was checked against public authority guidance:
- Australian Maritime Safety Authority: Under-keel clearance management — UKC definition and dynamic factors
- Connecticut DEEP Boater's Guide — bridge clearance, changing water levels, and vessel air draft
- UK government: Lydney Harbour — example of operator-specific maximum LOA, beam, and draft restrictions