Dock Line Length and Diameter Calculator

Estimate a transient dock-line kit from boat length: bow and stern line lengths, spring line lengths, total rope, and a starting diameter in inches and millimetres.

A starting estimate, not a mooring load calculationBoat displacement, windage, wakes, current, surge, water-level range, line construction, chafe, cleats, and dock geometry can require longer, larger, or additional lines. Follow line, vessel, and marina guidance.

Boat and line kit

Boat and conditions

The diameter result is shown in inches and millimetres.

Use LOA, not waterline length.

The exposed profile is still not a storm rating.

Adds working length before rounding for purchase.

Lines to carry

Combined count; usually at least one bow and one stern line.

Usually a forward and an aft spring for alongside docking.

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Dock-line plan

Enter the boat and kit details to calculate.

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What the calculator does

For a versatile transient kit, the calculator starts each bow or stern line at two-thirds of LOA and each spring line at one LOA. It applies your allowance, then rounds each purchase length up to the next whole foot or half metre.

End line = LOA × 2 ÷ 3
Spring line = LOA
Purchase length = guideline × (1 + allowance)

Diameter uses West Marine's published recreational table through 72 ft: 3/8 in through 27 ft, 7/16 in at 28–31 ft, 1/2 in at 32–36 ft, 5/8 in at 37–45 ft, 3/4 in at 46–54 ft, 7/8 in at 55–63 ft, and 1 in at 64–72 ft.

How to use the estimate

  1. Enter the boat's true length overall and choose feet or metres.
  2. Use the standard profile for a baseline; choose exposed/heavy to step up one standard diameter.
  3. Set the number of end and spring lines you want in the carried kit.
  4. Use the allowance for cleating, knots, handling, and unfamiliar dock geometry.
  5. Verify fit through chocks and on cleats, working strength, stretch, chafe protection, and actual conditions.

Example: 36-foot boat

At the published starting size, a 36 ft boat maps to 1/2 in dock line. With 10% allowance, each bow or stern line starts at 24 ft and rounds from 26.4 ft to a 27 ft purchase length; each spring line rounds from 39.6 ft to 40 ft.

2 × 27 ft end lines + 2 × 40 ft spring lines = 134 ft total purchased line

Ready-made dock lines come in fixed lengths, so select the next available product length that meets or exceeds the calculated per-line figure.

Dock line FAQs

How long should bow and stern dock lines be?

For transient lines carried aboard, about two-thirds of LOA per bow or stern line is a common baseline. Carrying longer lines adds flexibility at unfamiliar docks. Permanent lines should be fitted to the actual berth.

Why are spring lines longer?

Spring lines lead fore-and-aft to limit surging. A line approximately equal to LOA provides a useful transient starting length and more favorable geometry than a short breast line.

Should I choose the exposed/heavy profile?

Use it as a screening choice if the vessel is heavy for its length or the berth is exposed to wakes, current, or surge. It simply advances one standard diameter and does not calculate load or guarantee suitability.

Is larger line always safer?

No. Larger line may be stronger and resist wear longer, but it can stretch less, may not fit chocks or cleats correctly, and can transfer higher shock loads. The complete line-and-hardware system must be compatible.

What material is normally used for dock lines?

Nylon is commonly recommended because its stretch helps absorb dynamic loads. Construction, condition, wet strength, splices, and chafe protection also matter.

Can I use this for permanent or storm dockage?

Not as a final plan. Permanent and storm arrangements must account for exact berth geometry, water-level change, surge, exposure, redundancy, chafe, attachment strength, and marina or emergency guidance.

Limits and safety disclaimer

  • This calculator does not estimate wind, wave, wake, current, surge, or impact loads and does not establish a safe working load or breaking strength.
  • It does not inspect line age, UV damage, abrasion, splices, knots, chafe gear, cleats, chocks, pilings, dock hardware, or backing structure.
  • The published diameter table ends at 72 ft. Larger boats receive a rule-of-thumb extrapolation and require vessel-specific professional or manufacturer guidance.
  • Local tides and water-level changes can demand a different line arrangement. Recheck lines and fenders as conditions change.

Safety disclaimer: This is an educational equipment-planning estimate, not seamanship, mooring engineering, or storm-preparation advice. Use sound attachment points, protect every chafe point, inspect lines regularly, and follow the boat, line, marina, and local-authority guidance for the actual berth and forecast.

Methodology and sources

Last reviewed: August 1, 2026. Length guidance and the diameter table were checked against current recreational boating references:

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