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: Length x Beam x .85
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.