Skip to content
Free tool · Docking

What size fenders and dock lines does your boat need?

Two answers from one number. Enter your boat length, type, and berth, and get the fender diameter and how many to hang, plus the dock-line diameter and the length to cut your bow, stern, and spring lines. It runs the published size-to-boat charts so you can buy once and dock with confidence.

Your boat

A planning guide using the published fender and dock-line charts. Confirm against your fender maker's chart, and size up for a rough berth.

How to size fenders and dock lines

The calculator above is just this six-step method. You can run it by hand.

Step 1: Size the fender by boat length, then by type

The published fender charts (Polyform, Taylor Made, West Marine) size a cylindrical fender at roughly one inch of diameter for every four to five feet of boat, with a sensible floor for small boats. A 25 ft boat lands around a 6 inch fender, a 35 ft boat around 8 inches, and a 45 ft boat around 10 to 12 inches. Round ball fenders run about double that diameter for the same boat, and a pontoon usually wants its own rail or transom fenders rather than cylinders, so fix the type before you read a number.

Step 2: Hang one fender per 10 ft, and never fewer than three

The working rule is one fender for every ten feet of boat length on the side you tie up, with a minimum of three so the bow, beam, and quarter are all covered. A 30 ft boat wants three, a 40 ft boat four. Carry a couple of spares beyond that: you double up when you raft alongside another boat, and you want one to spare if a piling sits in an awkward place.

Step 3: Bump up a size when the berth is rough

The chart assumes a calm slip with a clean floating dock. An exposed or tidal berth that works the boat against the dock, rafting alongside in a swell, or a heavy or high-freeboard boat all ask more of the fenders: add one to the count and lean to the larger end of the diameter range, or step up to the next fender size. Oversizing a fender costs a few dollars and protects the gelcoat; an undersized fender that rolls out from between the hull and a piling protects nothing.

Step 4: Size the dock line by diameter first

Dock lines are sized by rope diameter, and nylon three-strand is the standard because it stretches to absorb surge and shock. The charts step the diameter up by boat length: about 3/8 inch up to 27 ft, 1/2 inch to about 31 ft, 5/8 inch to about 36 ft, and 3/4 inch into the mid-forties. Nylon, not polypropylene, which does not stretch and degrades in sun. Step up one diameter for a heavy boat, an exposed or tidal berth, or a line you leave on permanently.

Step 5: Cut the lengths to the job: bow and stern, then springs

Bow and stern lines run about two thirds of the boat's length each, and the spring lines, which stop the boat sliding forward and aft in the slip, run about the full length of the boat. A breast line, if you use one, is about half the boat length. So a 30 ft boat wants roughly 20 ft bow and stern lines and 30 ft springs. Buy them pre-spliced with an eye for the cleat, or splice your own, and whip the bitter ends.

Step 6: Carry four lines, six for a home berth

The minimum working set is four lines: a bow, a stern, and two springs, which is enough to hold the boat square in a calm slip. For a permanent berth or anywhere the wind and current shift, carry six (a bow and stern each side, plus two springs) so you can rig for any wind without re-leading a single line. Add chafe gear wherever a line crosses a chock or rail; chafe, not strength, is what parts a dock line.

Fender size chart, by boat length

Cylindrical fender diameter, the convergent published charts (Polyform, Taylor Made, West Marine). Round ball fenders run about double the diameter for the same boat. Hang one fender per 10 ft of length, minimum three, and lean to the larger end for an exposed berth.

Boat length Cylindrical fender diameter
Up to 20 ft 4 to 5 in
21 to 25 ft 6 to 6.5 in
26 to 35 ft 8 to 8.5 in
36 to 45 ft 10 to 12 in
46 to 60 ft 12 to 16 in
61 ft and up 16 in and up

Dock-line size chart, by boat length

Nylon three-strand diameter (Discover Boating and Better Boat charts). Bow and stern lines run about two thirds of the boat length, spring lines about the full length. Step up one diameter for a heavy boat, a tidal or exposed berth, or a permanent line. This chart runs on the heavier, conservative side; some makers (West Marine, Mercury) size about one rope size lighter using a 1/8 inch per 9 ft rule, and heavier line is never wrong for docking.

Boat length Line diameter (nylon)
Up to 27 ft 3/8 in
28 to 31 ft 1/2 in
32 to 36 ft 5/8 in
37 to 45 ft 3/4 in
46 to 54 ft 7/8 in
55 to 63 ft 1 in
64 ft and up 1-1/8 in

Now buy the right fenders

You have your sizes. Here is what we'd actually hang and tie with.

How we are paid: some links above go to Amazon, and if you buy through them we earn a commission at no extra cost to you. It does not change what we recommend.

Common questions

What size fenders do I need for my boat?

Size by boat length using the published charts: a cylindrical fender runs about one inch of diameter for every four to five feet of boat, so a boat up to 20 ft wants roughly a 4 to 5 inch fender, a 25 ft boat about 6 inches, a 35 ft boat about 8 inches, and a 45 ft boat about 10 to 12 inches. Round ball fenders run about double that diameter for the same boat. Lean to the larger end for a heavy or high-freeboard boat, or an exposed or tidal berth.

How many fenders do I need?

Hang one fender for every ten feet of boat length on the side you tie up, with a minimum of three. So a boat up to 30 ft wants three, a 40 ft boat four, and a 50 ft boat five. Carry a couple of spares beyond that, because you double up when you raft alongside another boat and you want one in reserve for an awkward piling.

What size dock lines do I need?

Dock lines are sized by diameter, in nylon three-strand. The standard chart runs about 3/8 inch for boats up to 27 ft, 1/2 inch to about 31 ft, 5/8 inch to about 36 ft, 3/4 inch into the mid-forties, and 7/8 to 1 inch above that. Step up one diameter for a heavy boat, an exposed or tidal berth, or a line you leave on the dock permanently.

How long should my dock lines be?

Bow and stern lines should be about two thirds of your boat's length each, and spring lines about the full length of the boat. A 30 ft boat wants roughly 20 ft bow and stern lines and 30 ft springs; a 40 ft boat wants about 27 ft bow and stern lines and 40 ft springs. Carry four lines as a minimum (bow, stern, and two springs) and six for a permanent berth.

Should dock lines be nylon or polypropylene?

Nylon, and three-strand or double-braid. Nylon stretches under load, which absorbs the surge and shock that would otherwise snatch at your cleats. Polypropylene does not stretch, it floats in a tangle, and it breaks down in sunlight, so it belongs on a dinghy painter, not on dock lines. Whatever you use, add chafe protection wherever the line crosses a chock or rail, because chafe is what parts a line, not a lack of strength.

What is a spring line and do I need one?

Spring lines run fore and aft along the dock rather than out to the side, and they stop the boat surging forward and backward in the slip while the bow and stern lines hold it off the dock. Yes, you need them: a boat held only by bow and stern lines walks forward and back with every wake and gust. Run them about the length of the boat, and for a permanent berth carry two so the boat is pinned from both directions.

Do the fender and line sizes change for a tidal or exposed berth?

Yes. The base chart assumes a calm slip on a floating dock. A tidal berth, a fixed dock the boat rides up and down against, or an exposed spot with chop and wakes all work the gear harder. Add a fender and lean to the larger diameter, step the dock line up one size, and add chafe gear at every chock. This tool bumps those numbers for you when you pick an exposed or rafting berth.

Sources & further reading

The sizing here follows the published fender and dock-line charts. Verify against the source if your boat sits on a band edge.

Disclaimer

This calculator and page are provided for general informational purposes only and reflect the accepted published fender and dock-line sizing charts as of 2026. They are a starting point for a calm-water slip, not a guarantee that a given fender or line will protect a particular boat in any condition. Loads vary with the wind, current, tide, wake, and how the boat is tied, and securing a boat is your responsibility. Lean toward sizing up for an exposed, tidal, or unattended berth, add chafe protection at every chock, and confirm against your fender maker's own chart before you buy. Sorted Gear accepts no liability for any loss or damage arising from reliance on this information. Last reviewed June 2026.

The Dispatch

New picks, when we publish them. No filler.

One short email when a guide goes up or a trip report is worth your time. Unsubscribe in one click.