Dock Lines & Fenders Calculator (Line Diameter, Lengths, Fender Size & Count, Cleat Size)

How it works

line diameter: ≈ 1/8″ per 9 ft of LOA (3/8″ to 27 ft, 1/2″ to 36 ft, 5/8″ to 45 ft, 3/4″ to 54 ft…), one size up for heavy, exposed or storm · bow/stern lines ⅔ LOA, springs = LOA · fender diameter ≈ 1″ per 4–5 ft LOA; count ≈ LOA/10 + 1 (min 3) · cleat length ≈ 16 × line diameter

Dock lines take shock loads from wakes, surge and wind, which is why nylon (which stretches 10–15%) is the material and why the size steps up with boat length rather than being calculated from a static pull. The standard rules of thumb used by marinas and chandlers are collected here: diameter by length, bow and stern lines two-thirds of the boat, springs the length of the boat, fenders sized to the length and numbered to cover the hull where it meets the dock, and cleats long enough to take the line without pinching. Exposed berths, heavy boats and storms move everything up a size and double the count.

Worked example

A 34 ft cruiser in a protected slip needs 1/2″ (12 mm) nylon: two bow/stern lines of about 23 ft and two springs of about 34 ft, four 8″ × 22″ fenders and 8″ cleats. Alongside a tidal wall it would take six lines with four springs; for a hurricane, eight to twelve lines of 5/8″, all doubled to separate cleats with chafe gear.

Frequently asked questions