The Tool Desk
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Boat fender sizing chart
The following chart reproduces the general leisure-boat guidance published by West Marine and the same size bands reported by Lippert. Cylindrical diameter is the most useful starting measurement for ordinary docking. Round and flat fenders use different dimensions and are selected by contact geometry rather than by converting directly from cylindrical diameter.
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| Boat length | Cylindrical diameter | Round diameter | Flat size |
|---|---|---|---|
| Under 10 ft | 3 in | Not recommended | Small |
| 10–15 ft | 3–4 in | 9 in | Small |
| 15–20 ft | 5–5.5 in | 12 in | Small |
| 20–25 ft | 6–6.5 in | 15 in | Medium |
| 25–35 ft | 8–8.5 in | 18 in | Medium |
| 35–50 ft | 10.5–12 in | 21 in | Large |
| 50–60 ft | 12 in | 27 in | Large |
| 60–72 ft | 15–24 in | 27 in | Large |
West Marine’s guide, last updated August 10, 2026, describes the chart as a general selection aid. It is not an engineering calculation for commercial berthing energy. Lippert’s May 3, 2023 guidance publishes the same broad size bands.
How to turn the chart into a real setup
- Find the boat-length range. Use the length figure supplied for your boat to select an initial cylindrical diameter, or the corresponding round or flat category if that shape better suits your berth.
- Identify where contact actually occurs. Look at the dock face, pilings, bulkhead, neighboring boat and the part of your hull that reaches them. A fender that hangs below or above the contact zone cannot protect the hull effectively, even when its diameter is adequate.
- Account for the boat’s mass. Length does not describe how much energy the boat can transfer into a dock. A heavy boat at the same length as a light boat places greater demands on the fender, particularly when it arrives with momentum or moves in a surge.
- Account for movement and exposure. A sheltered floating dock has different demands from an exposed slip subject to chop, wake or tidal movement. West Marine advises choosing the next size up when you are uncertain; treat that as a rule of thumb, not a guarantee.
- Check the individual product specification. Manufacturers may publish a recommended boat-length range, dimensions, line requirements and inflation pressure for a particular model. Those instructions take precedence over a generic chart.
Why diameter matters more than length alone
A larger diameter creates a thicker cushion between the hull and the dock and gives the fender more room to deform before hard contact. The length-based rule captures some of that need, but it cannot account for every combination of boat weight, hull shape and berth movement. Two boats of equal length can therefore need different protection.
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- Boat weight: A heavier hull increases the load the fender must absorb. If your boat is unusually heavy for its length, use the chart as a lower boundary and consider a larger diameter.
- Hull contact height: The fender must be large enough to cover the part of the hull that meets the dock. A narrow fender can leave the rub rail, freeboard or lower hull exposed when the boat heels or the water level changes.
- Exposure: Chop, wake, tidal surge and an exposed slip can repeatedly load a fender. Additional fenders or a larger size may be appropriate instead of relying on one fender at each station.
- Berth geometry: Pilings, gaps, high bulkheads and irregular dock faces can make shape and placement as important as diameter.
Choose a fender shape for the contact situation
Cylindrical fenders
Cylindrical fenders are the general-purpose choice for many boats. They can be hung vertically along a flat dock face and provide a continuous contact surface as the boat moves slightly along the dock. Use the diameter ranges in the chart as your initial selection, then adjust for weight and exposure.
Round fenders
Round fenders provide a broad circular cushion and are often useful where a concentrated point of contact needs more coverage. The chart lists round diameters from 9 inches for 10–15-foot boats through 27 inches for boats 50 feet and longer. Do not assume that a round fender with the same nominal size as a cylindrical model offers the same contact geometry; compare the actual diameter and the manufacturer’s boat-length recommendation.
Flat fenders
Flat fenders occupy less projection from the hull and can suit particular dock or storage constraints. The chart categorizes them as small for boats under 20 feet, medium for 20–35 feet and large above 35 feet. Select a flat model only after checking that its face covers the point where your hull meets the dock.
Specialized hulls and arrangements
Pontoons and other specialized hull forms may not present the same contact surface as a conventional deep-V or round-bilge boat. When a product is designed for a particular hull, use its stated dimensions and application guidance rather than converting the cylindrical rule yourself.
How many boat fenders do you need?
Quantity guidance varies because the sources count in different ways. West Marine’s rule is one fender per 10 feet of boat length, plus one. Its examples call for three fenders on boats up to 20 feet and four on boats from 20 to 30 feet. Lippert describes one fender per 10 feet of waterline per side, with at least three. These are planning rules, not a universal standard.
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| Guidance | Counting basis | Published example or minimum |
|---|---|---|
| West Marine | One fender per 10 ft of boat length, plus one | Three for boats up to 20 ft; four for 20–30 ft |
| Lippert | One fender per 10 ft of waterline per side | At least three |
Use the rule that matches how your berth is arranged, then add protection at every likely contact point. A boat that can touch both sides of a narrow slip may need fenders on both sides, while a boat against one fixed dock face may concentrate its working fenders on that side and keep others ready for maneuvering.
Where to place and how to hang fenders
Cover the bow, stern and middle
Place fenders near the bow and stern and amidships where the hull is most likely to meet the dock. The exact stations depend on your boat’s shape and the berth. Move a fender if it hangs in open water while an unprotected rub rail is touching the dock.
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Set the height to the actual contact zone
On a flat dock face, vertical hanging suits most situations. Adjust the line so the fender’s center covers the area where the hull presses against the dock, not merely the waterline at rest. Water level, loading and boat movement can change that height, so use fender lines or adjusters when frequent correction is needed.
Use horizontal hanging when the movement is vertical
Horizontal hanging can help where a boat rises and falls against a high dock or bulkhead. A horizontally oriented cylindrical fender can cover more vertical hull area in that arrangement. Check that it cannot roll away from the contact point as the boat moves.
Protect against pilings
Individual fenders can be pushed into the gap between pilings, leaving the hull against bare timber or hardware. A fender board spanning two or more pilings distributes the load and prevents a fender from disappearing between them. Secure the board and fenders so they remain aligned when the boat surges.
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Match the setup to your dock
Flat floating dock
Start with cylindrical fenders hung vertically at the bow, stern and amidships. Because a floating dock follows the water level, set the fenders for the current contact height and check them again after loading the boat or changing the water level.
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Fixed high dock or bulkhead
If the hull moves up and down against a tall vertical face, use horizontal hanging or another arrangement that keeps cushioning across the full travel of the hull. A fender that only covers one narrow band can allow the boat to ride over it.
Piling dock
Use a fender board when gaps between pilings could trap individual fenders. Position the board where the hull will meet the pilings and verify that its edges cannot damage the boat.
Exposed slip or tidal berth
Chop, wake and tidal surge increase movement and repeated loading. Start at the charted diameter, then size up or add fenders at the stations that experience the most motion. Secure every line so the fenders cannot swing out of position between impacts.
Inflation and product-specific instructions
Do not apply one model’s pressure recommendation to every inflatable fender. Taylor Made lists 2 PSI as the recommended inflation for its Tuff End line; that is a product-specific specification. Check the manual or product label for your exact fender, use the stated pressure method and inspect the fender for leaks or deformation before relying on it.
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Inflation is only one part of fit. Confirm the actual outside diameter, overall length, eye or rope arrangement and the boat-length range stated for the model. A nominal “8-inch” fender that measures differently when inflated can alter the clearance you expected from the chart.
Worked sizing examples
20-foot boat on a sheltered flat dock
The chart places a 20–25-foot boat at 6–6.5 inches for cylindrical fenders, 15 inches for a round fender or a medium flat fender. A three-fender starting quantity follows West Marine’s example for boats up to 20 feet; place them at the bow, stern and amidships, then observe whether any additional contact occurs. If the boat is heavy for its length or the dock sees wake, move to a larger option or add protection rather than treating the chart as a hard limit.
30-foot boat in an exposed slip
A 25–35-foot boat starts at an 8–8.5-inch cylindrical diameter, an 18-inch round diameter or a medium flat category. West Marine’s length rule gives four as the starting quantity for a 20–30-foot boat. Because this berth is exposed, inspect the highest-load contact points and consider a larger or additional fender. Keep the fenders aligned with the hull as wake and surge move the boat.
18-foot boat alongside pilings
The 15–20-foot row calls for a 5–5.5-inch cylindrical fender, a 12-inch round fender or a small flat fender. Three fenders provide the basic West Marine example for a boat up to 20 feet. Add a fender board spanning the pilings so the fenders cannot be forced into the gaps, and set the board and lines at the actual rub-rail height.
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- Confirm the boat-length range and the fender’s actual diameter, not just its marketing name.
- Choose cylindrical, round or flat geometry based on the dock and hull contact point.
- Decide whether the berth requires protection on one side or both.
- Plan stations at the bow, stern and amidships, adding coverage where movement concentrates.
- Use fender lines or adjusters if the height changes with tide, loading or dock design.
- Use a fender board for pilings that can trap individual fenders.
- Read the manufacturer’s inflation and line instructions for the exact model.
- Balance protection against handling and storage: a larger fender offers more cushion but is harder to move and stow.
Common sizing mistakes and fixes
Choosing by length only
Problem: The fender matches the chart but the boat still contacts the dock hard. Fix: Reassess boat weight, exposure and the actual contact height, then size up or add a fender where the load occurs.
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Using too few fenders
Problem: The bow or stern swings beyond the protected stations. Fix: Apply a quantity rule as a starting point and add coverage for the berth’s real movement. Lippert’s minimum of three is not a promise that three will cover every side of every boat.
Hanging at the wrong height
Problem: The fender floats below the rub rail or rides above the dock edge. Fix: Adjust the line or use an adjuster so the fender’s center remains at the contact zone as water level and loading change.
Letting a fender fall between pilings
Problem: A piling gap captures the fender and exposes the hull. Fix: Span the pilings with a properly secured fender board.
Generalizing an inflation number
Problem: A fender is over- or under-inflated because a pressure figure from another model was used. Fix: Follow the exact product’s instructions; the 2 PSI figure published for Taylor Made Tuff End fenders is not a universal setting.
What the sizing guidance can and cannot tell you
The West Marine and Lippert charts provide practical starting ranges for leisure boats. They do not measure impact energy for a particular hull, dock or weather condition, and they do not replace the manufacturer’s specification for a specific fender. The final setup depends on the boat’s mass, hull shape, berth geometry, exposure and the way the fenders are secured.
West Marine summarizes its sizing advice with the instruction, “When in doubt, choose the next size up.” Used in context, that means selecting a larger fender when the boat, dock or conditions make the chart’s minimum uncertain—not assuming that any larger fender will solve a badly positioned line or an unprotected contact point.
For additional product dimensions and line guidance, consult the maker’s documentation for the model you intend to use. Recheck the setup whenever you change berths, add equipment, alter the boat’s loading or encounter more wake and surge than usual.
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