Estimate boat-trailer winch strap length from bow-eye reach, minimum drum wraps, drum diameter, and spare tail. Path geometry — not a winch load rating or retail strap size picker.
Use when replacing a winch strap and you can measure the distance from the winch drum to the bow eye at the furthest launch position you use.
Enter reach, minimum wraps left on the drum (often ≥3), drum OD, and optional tail.
Not a substitute for measuring the old strap, ramp-specific reach, or maker minimum wrap rules.
Strap must cover the reach plus webbing that stays on the drum, plus spare. Wrap length uses circumference × wraps (drum OD converted to feet).
wrap_ft = wraps × π × (drum_od_in / 12)
strap_ft = reach_ft + wrap_ft + tail_ft
Not a winch capacity calculator.
Defaults: reach 18 ft, wraps 3, drum 3 in, tail 2 ft → wrap = 3 × π × 0.25 ≈ 2.36 ft → strap ≈ 22.36 ft.
Reach 22, wraps 4, drum 2.5 in, tail 1 → ≈ 25.62 ft.
Assumptions: single-layer wrap estimate; drum OD is the winding diameter; reach is the longest you need.
Limits: multilayer pile-up, bow stops short of the winch, and hinged bow rollers change the real path — measure with a tape/hose when unsure.
Responsibility: planning aid only — not a winch WLL stamp.
Fully stripping the drum puts the load on the strap anchor alone. Keeping wraps spreads load on the drum.
Use the diameter where the strap actually winds. A full drum of webbing is larger — this calculator is a minimum-length estimate.
Capacity calculators size pull rating vs boat weight. This page only sizes strap length.
Yes — buy the next common length (often 20 or 25 ft) after you compute the minimum.