Antifouling Bottom Paint Calculator – Coverage, Coats & Cans

Estimate underwater hull area, paint per coat, total antifouling volume, and whole cans to buy. Use a measured wetted area when available, or select a clearly labeled approximation. Inputs remain in this browser tab.

Use the exact product label and technical data sheetCoverage, film thickness, coat count, compatible substrates, preparation, protective equipment, recoat time, launch window, and disposal rules vary by product and jurisdiction. The calculator estimates quantity only.

Hull and paint inputs

Underwater area

Changing units converts all physical quantities.

A broad LOA-by-beam approximation used in manufacturer guidance.

Use the hull length requested by this method.

Optional allowance for separately assessed appendages or omitted surfaces.

Product and coating plan

Example only. Replace with the exact product's stated coverage at the required film thickness.

Follow the product's coating schedule.

For separately planned waterline, leading-edge, keel, or rudder reinforcement.

User-selected allowance for tray, roller, transfer, surface, and application losses.

Enter the actual purchasable can or kit volume.

Inputs are processed only in your browser and are not added to analytics events.

Paint quantity result

Enter the hull, product, and coating-plan details to calculate.

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Underwater hull-area formulas

Choose the formula whose named dimensions and hull profile fit the available manufacturer guidance:

  • General: 0.85 × hull length or LOA × beam.
  • Long keel: LWL × (beam + draft).
  • Medium draft: 0.75 × LWL × (beam + draft).
  • Fine/racing: 0.50 × LWL × (beam + draft).

These are geometric approximations, not measurements. Hull form, multihulls, appendages, transoms, waterline shape, and loaded condition can materially change area. Prefer measured or designer-supplied wetted area.

Paint, coats, and cans formula

Paintable area = base area × (1 + additional area %)

Equivalent coats = full coats + extra-coat area %

Paint before loss = paintable area × equivalent coats ÷ labeled coverage

Planning volume = paint before loss × (1 + loss %)

Cans = round up(planning volume ÷ package size)

The extra-coat percentage represents one additional coat over that fraction of the total area. For example, two full coats plus an extra coat over 20% of the area equals 2.2 area-equivalent coats.

How to use this antifouling calculator

  1. Use a measured underwater area if the designer, yard, or a reliable takeoff provides one. Otherwise select the approximation that matches the dimensions you can verify.
  2. Add only the percentage of separately assessed appendage or other paintable area omitted from the base area. Do not double-count surfaces already in a measured figure.
  3. Copy the theoretical coverage from the current label or technical data sheet for the exact product, application method, and required film thickness.
  4. Enter the specified full coats and, if applicable, the fraction of the bottom receiving one extra high-wear coat.
  5. Choose a defensible loss allowance, enter the package's actual volume, and buy at least the whole-can result. Confirm preparation, compatibility, safety, drying, launching, and disposal instructions before work.

Example: 30 ft boat, two coats

The general approximation gives 0.85 × 30 ft × 10 ft = 255 sq ft. Two full coats plus one extra coat over 20% of the bottom equals 2.2 equivalent coats, or 561 coat-sq-ft.

At an example coverage of 400 sq ft/US gal, that is 1.4025 US gal before losses. A user-selected 10% loss allowance raises the planning quantity to 1.54275 US gal. With 1 US gal cans, the purchase quantity rounds up to 2 cans. The 400 sq ft/US gal value is arithmetic for this example, not a universal product specification.

Frequently asked questions

How much antifouling paint do I need?

Divide the paintable underwater area by the exact product's labeled coverage to get volume per coat. Multiply by full coats plus any partial-area extra coat, then apply the chosen loss allowance. Divide by package size and round up to whole cans.

How does the calculator estimate boat bottom area?

The general method uses 0.85 × length × beam, following a common manufacturer approximation. Optional International Yacht Paint methods use LWL × (beam + draft) with a factor of 1.00, 0.75, or 0.50 for the named keel and hull profiles. A measured area is preferable.

Should I use LOA or waterline length?

Use the dimension named by the selected method. The general 0.85 × length × beam formula is published using hull length or LOA. The keel-profile methods are published using loaded waterline length (LWL). Substituting one for the other changes the estimate.

What coverage should I enter?

Enter the theoretical coverage for the exact product, application method, and specified film thickness from its current label or technical data sheet. Coverage differs substantially between products, and actual yield also changes with surface texture, equipment, applicator, temperature, and losses.

What does “area receiving one extra coat” mean?

It models one additional pass over only part of the bottom. If a product schedule calls for two full coats and an extra coat over 20% of the surface, the calculator uses 2.2 equivalent coats. Only enter this when the product instructions and your plan call for it.

Why are cans rounded up?

Paint is bought in whole packages. The planning volume is divided by the entered package size and rounded upward so purchased volume is not below the estimate. The displayed remainder is unopened-plus-usable volume in theory, not a promise of actual leftover paint.

Are my boat dimensions or paint choices uploaded?

No. Calculations run locally in the browser, and this tool does not send, store, or attach input values to analytics events.

Limits and antifouling safety disclaimer

  • Area formulas are approximations for conventional monohulls. They do not automatically model catamaran or trimaran hulls, propellers, shafts, struts, trim tabs, drives, rudders, unusual keels, or a separate boot stripe.
  • Labeled theoretical coverage assumes a specified wet or dry film thickness. Thinning, application method, surface profile, porosity, overspray, spillage, and retained paint can change practical yield.
  • This calculator does not select a paint, assess compatibility with an existing coating or substrate, determine legal availability, or prescribe coats, primers, thinners, PPE, ventilation, preparation, drying, launching, cleanup, or disposal.
  • Some copper-containing antifouling coatings are unsuitable for aluminum or underwater metals unless a manufacturer-approved system is used. Never infer compatibility from a quantity estimate.

Safety and environmental disclaimer: Antifouling coatings are regulated biocidal or pesticide products in many jurisdictions. Read and follow the current product label and safety data sheet, yard and marina controls, and applicable law. Contain sanding debris, paint, spills, and waste so they cannot enter soil, drains, or water. Use qualified professional help when the required preparation or application is outside your competence.

Methodology and sources

Last reviewed: August 2, 2026. Quantity arithmetic and area approximations were checked against coating-manufacturer guidance; label-compliance context was checked against UK HSE guidance:

The default inputs demonstrate the arithmetic only. They are not a recommendation for a product, coverage rate, coat count, loss factor, or package.

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