⚓ Hull Area Calculator
Estimate the wetted surface area of your hull — the area below the waterline you actually paint — from the waterline length, beam, draft, and block coefficient, in square metres and square feet.
⚓ Estimate Wetted Surface Area
What is a Hull Area Calculator?
It turns a few basic hull dimensions into an estimate of wetted surface area — the part of the hull that lives underwater and needs antifouling. Enter the waterline length, beam, and draft, pick a block coefficient for the hull form, and it returns the area in both square metres and square feet.
Use it to size an antifouling order, budget a bottom job, or sanity-check a yard quote. It's a planning estimate from a displacement-hull formula, so for a precise figure measure the surface directly or use the builder's published wetted area.
❓ Frequently Asked Questions
What is wetted surface area and why does it matter for painting?
Wetted surface area (WSA) is the area of the hull that sits below the waterline and stays in contact with the water. It's the surface you coat with antifouling, so knowing it lets you work out how much paint to buy and what a bottom job will cost. Get the area wrong and you either run short mid-job or waste money on paint you don't need.
How does this calculator estimate the hull area?
It uses a common displacement-hull approximation: WSA = LWL × (2 × draft + beam) × √(block coefficient). Waterline length, beam, and draft describe the hull's size, while the block coefficient captures how full or fine the underwater shape is. It's a quick estimate, not a substitute for a lines plan or a builder's figure.
What block coefficient should I use?
The block coefficient (Cb) is the ratio of the hull's underwater volume to the box that would enclose it. Fine-lined sailing yachts sit around 0.4–0.55, cruising yachts and motor cruisers around 0.55–0.7, and full-bodied displacement hulls and workboats 0.7 and up. If you don't know it, 0.65 is a reasonable default for a typical cruiser.
Is this estimate accurate enough to order paint?
It gets you a sensible ballpark for planning and paint budgeting, but real hull geometry — keels, skegs, rudders, and hull form — varies widely. For a precise order, measure the surface directly or use the builder's published wetted area, and always buy a little extra for touch-ups and losses.