In short
- Common boating guides size cylindrical fenders at about 1 in of diameter per 4 to 5 ft of boat length (about 2 in for round fenders).
- Use 1 fender per 10 ft of waterline, and treat 3 fenders per side as the bare minimum.
- Pilings roll and push fenders out of place. A fender board spanning pilings, or bumpers on the pilings, keeps the hull off them.
- Dock bumper sections and piling counts are our planning assumptions: no source publishes a spacing.
On this page
This boat fender size calculator is for Cape Coral boat owners who tie up at a canal dock. Enter your boat length and it returns a fender diameter range, how many fenders to hang on the dock side, and how many dock and piling bumpers to ask a dock builder about. It uses length rules of thumb only, with no prices or brands. It pairs with the hurricane dock line calculator, which sizes storm lines for the same boat.
En español: calculadora de defensas y topes para un muelle de Cape Coral
Example: a 38 ft boat with cylindrical fenders, pilings not counted, so the tool estimates them.
How the numbers are worked out
- Size the fender
Divide boat length by 5 for the low end and by 4 for the high end (1 in of diameter per 4 to 5 ft). Round fenders use twice that. Both ends round to the nearest half inch.
- Count the fenders
One fender per 10 ft of waterline, rounded up, never fewer than 3 per side. We add one roving fender, placed by a crew member at the last minute.
- Cover the dock edge
Cover the length of the boat along the dock edge with 4 ft bumper sections, rounded up. The section length is our assumption.
- Count the pilings
One bumper per piling the boat can touch. If you leave the count blank, the tool assumes one piling every 8 ft along the boat, plus one.
Fender size and count by boat length
| Boat length | Diameter (in) | Per side |
|---|---|---|
| 20 ft | 4 to 5 | 3 + 1 |
| 28 ft | 5.5 to 7 | 3 + 1 |
| 38 ft | 7.5 to 9.5 | 4 + 1 |
| 46 ft | 9 to 11.5 | 5 + 1 |
Cylindrical fenders, rounded to half inches. Retail size charts use set bands (for example 6 to 6.5 in for 20 to 25 ft boats), so your closest real size will differ a little. Round fenders: double the diameters.
What the rules do not cover
Fenders do not replace good docking. They absorb contact, not a boat pushed on the dock by wind or a storm. For hurricane prep use storm lines and chafe guards, not fenders alone: see the hurricane dock line calculator. Dock bumpers and piling guards go on the dock side, so ask the person building or repairing the dock to include them.
Fender checklist
- Widest point first. Put the middle fender at the boat's maximum beam, the others fore and aft.
- Just above the water. Too low and fenders dip and grow marine life; too high and they get pushed out.
- Watch the pilings. Fenders roll off them. Use a fender board or piling bumpers.
- Corners take the worst hits. Corner bumpers at the end of a dock are worth asking about.
- Check the label. Manufacturers print a size range on fenders. Match it to your boat length and weight.
Questions to ask a dock builder
- Can you fit bumpers to the dock edge and pilings? Ask where and how they are fastened.
- Are my pilings spaced for a fender board? See how many pilings a dock needs.
- Do I need a permit to change the dock? See the dock and boat lift permit checklist.
- How do I choose a contractor? See choosing a seawall contractor.
Next step
This is a planning estimate, not an engineering design. We are a referral service, not a contractor; we connect visitors with independent local providers. Use the form below to ask independent marine and seawall contractors about dock bumpers and piling protection.
Sources
- Boating Magazine, What to Look for When Buying Boat Fenders: 1 in of diameter per 4 to 5 ft (2 in for ball fenders), 1 fender per 10 ft of waterline, three as the bare minimum, fender boards for pilings.
- Discover Boating, Boat Fenders 101: two to four fenders per side, a roving fender, placement at the widest part of the hull and just above the waterline.
Frequently asked questions
What size fenders do I need for my boat?
A common rule of thumb is about 1 inch of diameter for every 4 to 5 feet of boat length for cylindrical fenders, and about 2 inches for round fenders. Retail charts use set size bands, so pick the closest size, and go one size up for a heavy boat or an exposed berth.
How many fenders do I need?
A common guide is one fender for every 10 feet of waterline, with three as the bare minimum per side. Many guides add a roving fender that a crew member places at the last minute. This calculator shows the count for one side and the total for both.
Where should I hang boat fenders?
Along the widest part of the hull, since that touches the dock first, and just above the waterline. On a fixed dock the tide changes the height, so you may need to adjust them.
Do I need dock bumpers as well as fenders?
Fenders hang on the boat and bumpers are fixed to the dock or pilings. Bumpers protect the dock edge and pilings and keep a fender from rolling off. No source gives a standard spacing, so the bumper and piling counts here are planning assumptions to discuss with a dock builder.
Do fenders protect a boat in a hurricane?
No. Fenders are for normal docking and wind chop. In a storm use properly sized storm lines, chafe guards and a plan for surge; see our hurricane dock line calculator.