How it is calculated
Length = boat length overall + outboard or swim platform beyond it + trailer tongue + clearance. Width = the wider of beam and trailer + room to walk. The standard size is the smallest of 10, 12 or 14 ft wide and 10 to 50 ft long (5 ft steps) that fits. Height: boat on the trailer must clear the door or roof with at least 2 inches to spare.
Checkable example
20 ft boat, 1.5 ft outboard, 4 ft tongue, 1 ft clearance → 26.5 ft; 8 ft beam on an 8.5 ft trailer + 1 ft → 9.5 ft. Space: 10 × 30 ft. 96 in high under a 120 in door: 24 in to spare.
Defaults are assumptions
- Outboard overhang, tongue length and trailer width vary a lot: measure yours.
- Unit sizes are nominal; a «10 × 30» door is usually narrower than 10 ft.