Antifouling Paint Coverage & Hull Surface Area Calculator
Estimate your boat's underwater hull surface area and calculate how much antifouling paint is needed based on coverage rate, number of coats, and an optional waste allowance.
Antifouling Paint Result
How do you calculate antifouling paint needed?
Once the underwater hull area is known, multiply the area by the number of coats and divide by the manufacturer's coverage rate.
$$Paint = \frac{Hull\ Area \times Coats} {Coverage}$$
A waste allowance can then be added:
$$Paint_{total} = Paint_{base} \times \left( 1 + \frac{Waste}{100} \right)$$
How do you calculate boat hull surface area?
The exact underwater surface area is best obtained from boat plans, CAD data, or a hull-surface measurement. For an early estimate, the calculator uses:
$$Area \approx LOA \times (2 \times Draft + Beam) \times Hull\ Form\ Factor$$
This is an estimation formula rather than a naval-architecture hydrostatic surface-area equation. The hull form factor adjusts the simple geometric estimate for the actual shape of the hull.
What is hull form factor?
A hull form factor is a simple correction used to make an estimated surface area closer to a real underwater hull. A value below 1.0 reduces the simple geometric estimate; the appropriate value varies with hull shape.
Example: antifouling paint calculation
Suppose a boat has an estimated underwater area of 500 ft², the paint covers 400 ft² per US gallon, and two coats are required.
$$Paint = \frac{500 \times 2}{400} = 2.5\ gallons$$
With a 10% waste allowance:
$$Paint_{total} = 2.5 \times 1.10 = 2.75\ gallons$$
You would therefore plan for approximately 2.75 gallons before considering the manufacturer's actual container sizes.
How many coats of antifouling paint should I apply?
The required number of coats depends on the specific antifouling product, application method, and manufacturer's instructions. Some systems also recommend an extra coat on high-wear areas such as leading edges, waterline areas, rudders, or keels.
What is antifouling paint coverage?
Coverage is the surface area that a specified amount of paint can cover at the recommended film thickness. It is usually given by the manufacturer in units such as m²/L or ft²/gal.
Always use the coverage rate printed in the technical data for the specific antifouling product rather than a generic coverage value.
Why does paint coverage change in practice?
Actual coverage can be lower than the theoretical value because of hull texture, rollers and brushes, spray losses, uneven application, temperature, viscosity, substrate condition, and the amount of paint left in application equipment.
Does hull shape affect antifouling paint quantity?
Yes. Two boats with the same length and beam can have different underwater surface areas. A deep or highly rounded hull generally requires more paint than a very flat hull of similar dimensions.
Does draft affect hull surface area?
Yes. A greater draft generally means more underwater hull surface, so the amount of antifouling paint required tends to increase.
What areas of a boat need antifouling paint?
Antifouling is normally applied to the underwater surfaces specified by the coating system, such as the hull below the waterline and, depending on the vessel and product, the rudder, keel, skeg, or other continuously immersed surfaces.
The exact coating boundary should follow the manufacturer's instructions and the boat's coating system.
Antifouling paint coverage chart
These are example calculations using a fixed coverage rate of 400 ft² per US gallon and two coats.
| Hull Area | 1 Coat | 2 Coats | 3 Coats |
|---|---|---|---|
| 200 ft² | 0.50 gal | 1.00 gal | 1.50 gal |
| 300 ft² | 0.75 gal | 1.50 gal | 2.25 gal |
| 400 ft² | 1.00 gal | 2.00 gal | 3.00 gal |
| 500 ft² | 1.25 gal | 2.50 gal | 3.75 gal |
| 600 ft² | 1.50 gal | 3.00 gal | 4.50 gal |
| 800 ft² | 2.00 gal | 4.00 gal | 6.00 gal |
| 1,000 ft² | 2.50 gal | 5.00 gal | 7.50 gal |
How much extra antifouling paint should I buy?
A small allowance can account for paint lost during mixing and application. The correct allowance depends on the hull, applicator, product, and application method.
$$Paint_{purchase} = Paint_{required} \times (1 + Waste)$$
For example, a 10% allowance means multiplying the base quantity by 1.10.
Can I calculate antifouling paint from boat length alone?
Length alone is not enough for an accurate surface-area calculation. Beam, draft, and hull shape also have significant effects. For an accurate result, use the actual underwater hull surface area.
Antifouling Paint FAQ
Calculate underwater hull area, multiply by the number of coats, divide by the manufacturer's coverage rate, and add an allowance for application waste if appropriate.
It depends on the boat's beam, draft, hull shape, number of coats, and the paint's coverage rate. Length alone is not enough for an accurate quantity.
Use actual hull data when available. For a preliminary estimate, LOA, beam, draft, and a hull-form correction can be used.
Coverage varies by product. Use the manufacturer's stated coverage rate, usually expressed in ft²/gal or m²/L.
Follow the specific antifouling manufacturer's instructions. The required number of coats varies between products and application systems.
Include any underwater surfaces that your coating system calls for. If your hull-area estimate excludes the keel or rudder, their areas should be added separately.