Dock Line Calculator

A 32 foot boat wants 21 foot bow lines, 32 foot springs, and half inch rope. Get the whole list before you go to the chandlery.

Dock Line Calculator gives you every line the boat needs, in length and diameter, from three measurements.

THE WHOLE SET, NOT ONE NUMBER Bow and stern lines at about two thirds of overall length. Spring lines at about the full length. Line diameter from an eighth of an inch for every nine feet, rounded up, with a practical minimum. One boat, one screen, all of it.

HEAVY WEATHER GETS ITS OWN FIGURE One size up on diameter when it is going to blow, shown alongside the normal size so you can see the difference before you buy. Most storm damage in a marina is chafe rather than breakage, and the app says so where it matters.

ANCHORING TOO Snubber length from bow freeboard, bridle legs from beam. The two figures people work out on the foredeck in the dark, already done.

A SHOPPING LIST YOU CAN TAKE TO THE COUNTER Every line with its length, diameter and quantity, in one list. Stop doing arithmetic at the chandlery.

FEET OR METRES, AND BOTH FOR DIAMETER Pick the units you think in. Diameter always shows in inches and millimetres together, because rope is sold both ways depending on where you are standing.

SAVE YOUR BOATS Keep the figures for each vessel you look after and load them back with one tap.

WORKS OFFLINE No account, no signal.

These are rules of thumb used widely in recreational boating, presented as starting points rather than standards. Your berth, your cleats, your conditions and your insurer may all want something different.