Measure centerline length from bow to transom and beam at the widest point. Add drop on both sides and both ends so the sheet ties below the gunwale. Support a ridge so rain leaves the cockpit instead of ponding. Do not size a boat as if it were a rectangular crate.
Boat covers fail from too little fabric to tie, too much unsupported fabric that holds water, and sawing on windshield corners and cleats.
Length, beam, and drop
- Put the boat on level ground or the trailer. Remove only the gear you will not store under the cover.
- Measure centerline length in a straight, level line from the farthest point of the bow to the transom. Taylor Made and Wholesale Marine both warn against running the tape up and over the windshield; that path is longer than the boat.
- Decide what stays under the tarp. Permanent windshields, consoles, rails, and a factory swim platform usually stay. An outboard is usually a separate motor cover unless you are buying a sheet that is meant to swallow it.
- Measure beam at the widest point of the hull, tape level, not wrapped over seats or rails. If you are unsure, measure in a few places and keep the largest number.
- Measure drop: from the gunwale or the highest rail down to the tie-down line on each side and at bow and stern. Drops can differ if the trailer winch post or a motor bracket gets in the way.
- If the cover will ride over a windshield, tower, or a ridge pole, measure that extra path separately. Sailrite’s boat-cover calculator takes length “on the center line over the top of the supporting structure” and a separate skirt depth below the rail. That is the geometry a rectangular tarp has to fake with extra fabric and a pole.
Planning formulas
Tarp length ≈ covered centerline + bow drop + stern drop
Tarp width ≈ beam + left drop + right drop
Add the extra centerline path if the sheet goes over a windshield, tower, or ridge pole.
Why a hull is not a box
The 3D-object formula on the parent sizing guide adds two full heights in each direction. That is a safe oversize for a crate. On a boat it buys fabric you then have to gather at the bow and along the sheer, and it still under-counts a windshield or a T-top.
Use length, beam, and drop. Then add only the extra path that actually exists: over the windshield, over a support pole, around a console. Two boats with the same length and beam still need different covers if one has a tower and the other does not. Wholesale Marine’s measuring guide is built around that point.
| Boat | Measure | What the box formula misses |
|---|---|---|
| Open skiff / jon boat | Centerline, beam, drop below gunwale | Little. This is the closest to a simple cover. |
| Runabout with windshield | Centerline to transom, plus path over the glass if the tarp goes over it | The windshield is a ridge and a chafe edge, not extra box height on all four sides. |
| Pontoon | Deck length and the full deck width, not a single hull beam | Width grows fast; furniture and rails sit above the deck. |
| Tower / T-top | Confirm the product is made for that profile | A rectangular tarp will not follow a tower. Specialty or custom is the usual answer. |
Drainage and ponding
A cockpit is a bowl. A tarp laid flat on it becomes a pond. Taylor Made tells owners to use support poles or a frame so the cover cannot sag. Covercraft’s Carver marine warranty is more specific: damage from pooled water may not be covered, and they recommend vented support poles or a cover-support system so water, snow, and ice run off.
Build a ridge along the centerline, high enough that water leaves at the sides. Retension after the first rain. A larger sheet without a ridge is not a drainage plan.
Chafe points
Wholesale Marine’s fit check is to pad windshields, cleats, and other sharp or projecting objects. Those are the spots a tarp saws through first: windshield corners, bow rails, rod holders, antenna bases, trailer winch posts. Pad them before you tension the cover. Tie below the gunwale so wind cannot get under the edge and turn the sheet into a file.
Leave a path for moisture if the boat will sit for weeks. A sealed wet cover traps condensation on seats and carpet. Dry the interior first when you can.
Worked example
A 16 ft runabout measures 16 ft bow to transom and 7 ft 6 in beam. You want 18 in of drop below the gunwale on all four sides, and the outboard stays on a separate motor cover. Minimum tarp length is 16 + 1.5 + 1.5 = 19 ft. Minimum width is 7.5 + 1.5 + 1.5 = 10.5 ft. A 20×12 ft stock sheet clears both.
If that same tarp must also go over a windshield that stands about 2 ft above the gunwale, measure the extra centerline path over the glass and add it to length. Do not add 2 ft of height twice the way you would for a box. Put a pole or the windshield itself under the ridge so the extra fabric becomes slope, not a pond.
Measuring: Taylor Made, “How to Pick the Right Boat Cover” (center-line length bow to transom, beam at the widest point; outboards and swim platforms not in the standard length). Wholesale Marine, “How to Measure Your Boat for a Cover” (level centerline, not over the windshield; pad projections; support against standing water). Length over a support and skirt depth: Sailrite boat-cover fabric calculator. Ponding and support poles: Taylor Made maintenance notes and Covercraft Carver marine warranty.
Frequently asked questions
Should a boat tarp be longer than the boat?
Yes. Include bow and stern overhang plus room to tie below the gunwale. A cover that ends exactly at the hull edge is hard to secure and easy for wind to peel back.
Can I use a rectangular tarp instead of a custom boat cover?
Yes for short-term storage, especially on a simple open boat. Custom and semi-custom covers fit windshields and hardware better for long outdoor storage or trailering. Taylor Made does not recommend a universal cover for trailering.
How do I keep a boat tarp from collecting water?
Support a ridge with a pole, board, or cover-support system so water runs off. Taylor Made and Covercraft’s Carver marine division both warn that standing water stretches fabric and can void cover warranties. A flat sheet on a cockpit becomes a pond.
Does the motor count in the length?
Usually no for a standard boat-cover measurement. Taylor Made and Wholesale Marine measure bow to transom and treat the outboard as a separate motor cover unless the product you are buying says otherwise. If your tarp must protect the motor, measure that full length and say so.
What material is best for a boat cover tarp?
For on-off use, a manageable PE or light coated fabric is practical. For long outdoor storage, stronger coated fabric and a real UV package last longer. The ridge and the chafe pads matter more than buying one size larger and hoping water runs off.
How this guide was prepared
We separate geometry and unit conversions from product claims, use exact standards where available, and flag specifications that vary by manufacturer. See our editorial policy for the review process.