A simple way to determine the amount of bottom paint you need is to figure the approximate surface area (in square feet) of your hull below the waterline. Here’s an easy formula to use:

Multiply the length of your boat's hull by its beam, then multiply again by 0.85. This formula provides the surface area, in square feet that needs to be painted. Then, check the coverage of the paint you've chosen.

Here are some approximate amounts of paint needed for typical boats and sizes. Keep in mind that actual square footage and coverage varies by boat and bottom paint type.

• 10' dinghy = 1 quart
• 14' outboard = 1-2 quarts
• 18' runabout = 2 quarts
• 20' sailboat = 3 quarts
• 24' runabout = 3-4 quarts
• 30' sailboat = 1.5 gallons
• 32' cruiser = 1.5-2 gallons
• 36' auxiliary = 2 gallons
• 40' sailboat = 2.5 gallons
• 45' sailboat = 3-3.5 gallons
• 50' sportfisher = 4 gallons

Because most antifouling paints have ablative or self-polishing properties, they wear away as the boat moves through the water, or as water moves beneath the boat. For this reason, you want to apply two coats in order to get sufficient protection from shell and weed fouling. It's also a good idea to apply an extra coat of paint to areas that see excessive wear, such as the leading edge of the keel, and at the waterline. Note that if you choose a hard modified epoxy paint (as opposed to an ablative paint), you only need to apply a single coat, as long as it covers completely. For specific film thicknesses per coat and recommended number of coats, refer to the manufacturer's directions on the product label.