Marine Paint Calculator: How Much Paint Do You Need?

A marine paint calculator estimates the amount of paint required to cover a boat’s hull, deck, or other surfaces based on dimensions you enter, such as length, beam, and draft. You input your boat’s measurements and the number of coats, and it returns the total surface area in square feet or square meters, plus the estimated paint volume in liters or gallons. This helps you budget accurately and avoid buying too little or too much paint.

Formula

The calculator uses a simplified hull surface area formula for a box-shaped hull:

Surface Area = 2 × (Length × Draft + Beam × Draft) + Length × Beam

Where:

• Length (L) = boat length at the waterline or overall length, in feet
• Beam (B) = maximum width of the boat, in feet
• Draft (D) = vertical distance from waterline to bottom of the hull, in feet

Paint coverage is typically around 300 square feet per gallon per coat for most marine paints (about 8 square meters per liter). So:

Paint Needed = (Surface Area / 300) × Number of Coats

This gives an estimate in US gallons. For metric, use 1 gallon ≈ 3.785 liters.

Worked Example

Using the default values: Length = 30 ft, Beam = 10 ft, Draft = 4 ft, Coats = 2.

1. Calculate the side areas: 2 × (30×4 + 10×4) = 2 × (120 + 40) = 2 × 160 = 320 sq ft
2. Add the bottom area: 30 × 10 = 300 sq ft
3. Total surface area = 320 + 300 = 620 sq ft
4. Paint for one coat: 620 / 300 = 2.07 gallons
5. For 2 coats: 2.07 × 2 = 4.13 gallons

The calculator shows 620 sq ft and 4.1 gallons (rounded to one decimal).

Why Use a Marine Paint Calculator?

Buying marine paint is not like buying house paint—marine coatings are expensive and often need to be applied in multiple coats. Underestimating means you run out mid-project, which can lead to uneven coverage or having to buy a different batch (color mismatch). Overestimating wastes money and leaves you with leftover paint that may not store well. A calculator gives you a solid starting number, which you can adjust for the specific paint brand’s coverage rate (usually printed on the can).

How to Choose the Right Paint Type

The calculator only tells you how much paint, not which kind. That depends on your boat’s material and usage:

• Bottom paint (antifouling) – for submerged hulls to prevent barnacles and algae. These are ablative (slowly wearing) or hard epoxy. Coverage is often 300-400 sq ft/gal.
• Topside paint – for above-water hull and deck, usually polyurethane or one-part acrylic. Coverage similar.
• Primer – needed on bare wood or metal. Coverage varies, often 200-300 sq ft/gal.
• Specialty coatings – like non-skid deck paint, which may have lower coverage due to texture.

Always check the manufacturer’s data sheet for the exact coverage rate and recommended number of coats.

Common Mistakes and How to Avoid Them

• Using overall length instead of waterline length – For bottom paint, use the waterline length, not the LOA (length overall). The calculator uses overall length, so adjust if your boat has a long overhang.
• Forgetting the keel and rudder – These add area. Add 5-10% to the result if your boat has a deep fin keel or large rudder.
• Ignoring surface texture – Rough or porous surfaces (old anti-fouling, bare wood) absorb more paint. Add 10-20% for first coats.
• Rounding up too little – Paint is sold in gallons or liters. If you need 4.1 gallons, you must buy 5 gallons (or 4 gallons and a quart).

Limitations of the Formula

This box-shaped model is a rough approximation. Real hulls are curved, V-shaped, or have chines, which changes the surface area. For a more accurate number, use a hull surface area calculator that asks for more dimensions, or use the “waterline length × (beam + draft) × 0.85� formula used by some painters. But for planning purposes, the box formula is within 10-20% for most monohulls. It is not suitable for catamarans (two hulls) or planing hulls with wide flat bottoms—those need a different approach.

Cost Considerations

While the calculator gives volume, cost depends on the paint brand and type. Bottom paint can range from $100 to $300 per gallon, while topside paints are similar. Primers are cheaper. A typical 30-foot sailboat needing 4 gallons of bottom paint might spend $400-$1200 just on paint, plus primer and thinner. Always add 10% for waste and touch-up.

Safety and Environmental Notes

Marine paints, especially antifouling, contain biocides (like copper) that are toxic to marine life. Check local regulations—some areas restrict certain paints. When applying, wear gloves, goggles, and a respirator if using solvent-based paints. Work in a well-ventilated area, and never dispose of leftover paint or wash water into the water or storm drains. Many marinas have specific disposal procedures.

Alternatives to a Calculator

If you prefer not to calculate, you can:

• Ask the paint manufacturer’s technical support—they often have online tools or can estimate from your boat’s specs.
• Use a professional painter’s rule of thumb: for a 30-foot boat, expect about 2 gallons of bottom paint per coat.
• Measure the actual hull with a tape measure and use a more detailed formula, or use a 3D model if you have one.

But a calculator is the quickest way to get a reasonable estimate before you start.

Frequently Asked Questions

How accurate is this calculator?

It is accurate to within about 10-20% for typical monohull sailboats and powerboats. The box model ignores curves, so it overestimates slightly for round-bottomed boats and underestimates for flat-bottomed ones. Always add 10% for safety.

Can I use it for deck paint?

Yes, but for decks you need the deck area, not the hull. Measure length × width of the deck, and use that as the “bottomâ€� area. The calculator’s formula includes the bottom, so you can just use the length × beam part.

What if my boat is a catamaran?

This calculator is not suitable. For a catamaran, calculate each hull separately using its own length, beam, and draft, then add the results. The formula still works per hull, but the total is the sum of both hulls.

How do I convert gallons to liters?

Multiply gallons by 3.785. For example, 4.1 gallons × 3.785 = 15.5 liters. Most marine paints are sold in liters outside the US.

Should I buy extra paint?

Yes, it’s wise to buy 10-15% extra to cover waste, spills, and touch-ups. You can also keep a small amount for future repairs, as long as it’s stored properly (sealed, cool, and dry).

Does the calculator account for primer?

No, it only calculates the topcoat. Primer is a separate coat with its own coverage rate. Add it manually if you plan to prime.

Related

• Exterior Paint Calculator: Square Feet Coverage
• Epoxy Paint Calculator: Coverage & Cost
• Emulsion Paint Calculator: How Much Paint You Need

in l | Boat length | ft | 30 | 0
in b | Beam (max width) | ft | 10 | 0
in d | Draft (hull depth) | ft | 4 | 0
in c | Coats | - | 2 | 1
out area | Hull surface area | sq ft | 2 * (l * d + b * d) + l * b | 0
out paint | Paint needed | gallons | (area / 300) * c | 1