Takeoff suggested bow, stern, breast, and spring line lengths from overall boat length (LOA). Based on common recreational rules of thumb — not a marina contract.
Use this when sizing bow, stern, breast, and spring lines from LOA before you buy or cut dock lines for a new berth.
Enter LOA (ft). The calculator returns planning lengths: bow/stern ≈ 0.67×LOA, breast ≈ 0.5×LOA, spring ≈ 1.0×LOA, plus a diameter rule-of-thumb.
Not a substitute for measuring actual cleat-to-cleat paths, surge in an exposed fairway, or marina rules that demand longer springs.
Common recreational guidance (including West Marine’s dock-line advisor): bow and stern lines about ⅔ of LOA, breast lines about ½ LOA, spring lines about 1 × LOA.
bow/stern ≈ 0.67 × LOA
breast ≈ 0.50 × LOA
spring ≈ 1.00 × LOA
Diameter rule of thumb often cited: ~⅛″ diameter per 9′ of boat length (step up for heavy displacement or exposed berths).
Tidal slips, high freeboard, and exposed docks may need longer or thicker lines. Nylon three-strand is the usual stretchy choice for dock lines. This page sizes length takeoff only.
| Approx LOA | Common nylon dia. (guide) |
|---|---|
| Up to ~27 ft | 3/8″ |
| ~28–31 ft | 7/16″–1/2″ |
| ~32–36 ft | 1/2″–5/8″ |
| ~37–45 ft | 5/8″–3/4″ |
Defaults: LOA 30 ft.
bow/stern ≈ 0.67×30 = 20.1 ft; breast ≈ 0.50×30 = 15.0 ft; spring ≈ 1.00×30 = 30.0 ft. Diameter thumb ≈ ⅛″ per 9′ LOA → 30/9 ≈ 3.33 → step to common ⅜–½″ class (confirm displacement).
| LOA | Bow/stern | Breast | Spring |
|---|---|---|---|
| 24 | 16.08 | 12.00 | 24.00 |
| 30 | 20.10 | 15.00 | 30.00 |
| 36 | 24.12 | 18.00 | 36.00 |
| 42 | 28.14 | 21.00 | 42.00 |
| 48 | 32.16 | 24.00 | 48.00 |
Rows: bow=0.67×LOA, breast=0.5×LOA, spring=LOA.
Assumptions: floating dock with typical cleat placement; LOA is the planning driver; diameter rule is a thumb, not a class rule.
Where it fails: Mediterranean lazy-lines, Mediterranean stern-to with long shore lines, and tidal ranges that need much longer springs.
Not a substitute: planning aid only — walk the cleats with a tape when the berth is weird.
Bow/stern need reach to outer cleats; breasts are shorter across the beam; springs run long to control surge.
Three-strand nylon stretches under load — useful in surge, but do not size so short that stretch is your only mooring.
Spring-line-length focuses on one spring geometry. This page is the LOA family set (bow/stern/breast/spring).
The ⅛″-per-9′ thumb is a starting class. Step up for heavy displacement or exposed berths.
Convert LOA to feet for these multipliers, or scale the same ratios in meters (results then in meters).