Size forward/aft springs from LOA × factor (commonly ~1.0–1.25). Optional breast length = LOA × breast factor.
Use this when you are sizing forward and aft spring lines from boat length overall (LOA) and a length factor, plus a little extra for splices, eye size, or tide range. Optional breast factor yields a companion breast-line length with the same extra.
Cleat positions, finger-pier geometry, and how far the boat sits from the dock change the real needed length — measure once in your slip after you have a starting number. Bow and stern dock-line lengths live on the dock-line length calculator.
This is layout arithmetic from your LOA and factors, not a dockmaster’s order, not load rating, and not a substitute for good cleats and chafe gear.
Spring length is LOA times the spring factor, plus extra feet. If breast factor is greater than zero, breast length is LOA times breast factor plus the same extra.
spring = LOA × spring_factor + extra
breast = LOA × breast_factor + extra (when breast_factor > 0)
Common spring factors cluster around 1.0–1.25 of LOA depending on slip layout; breast lines are often shorter fractions. Extra covers eyes, splices, and a tide cushion — it is not a load or stretch model (see dock-line stretch for elongation).
| LOA (ft) | Spring factor | Breast factor | Extra (ft) | Spring (ft) | Breast (ft) |
|---|---|---|---|---|---|
| 36 | 1.10 | 0.50 | 3 | 42.6 | 21.0 |
| 40 | 1.25 | 0.60 | 2 | 52.0 | 26.0 |
| 28 | 1.00 | 0.50 | 0 | 28.0 | 14.0 |
| 45 | 1.15 | 0 | 4 | 55.8 | — |
| 32 | 1.20 | 0.55 | 3 | 41.4 | 20.6 |
Rows were computed independently (spring = LOA×sf+extra, breast = LOA×bf+extra when bf>0) and match this page. Default example: 36 ft LOA × 1.1 + 3 → 42.6 ft spring · breast 21.0 ft.
Units: LOA and extra in feet; factors are dimensionless.
Where the formula fails: Med-moor stern-to with long passarelle runs, raft-ups, mooring pendants (different page), and any case where the fair lead path is much longer than LOA×factor. Diameter and working load are out of scope — see dock-line diameter.
Not a substitute: planning aid only — not a marina rulebook, not a rigger’s certificate, and not advice from a dockmaster who knows your slip.
Springs control fore-aft surge; breasts pull the boat toward the dock. Bow and stern lengths are on the dock-line length page. This calculator sizes springs (and optional breasts) from LOA×factor + extra.
Many skippers start near 1.0–1.2 × LOA for springs and ~0.5 × LOA for breasts, then adjust after a real dock-up. Your cleat geometry wins over a default.
No. Extra feet are for eyes/splices/tide, not nylon elongation under load. Use the dock-line stretch calculator for percent stretch.
Convert LOA and extra to feet before entry, or convert the result afterward. Do not mix meter LOA with a “feet” mental model of the extra field.
The script only reports breast length when breast factor is greater than zero, so you can blank or zero it when you only want springs.