The "1/15th Rule" Explained
The most common formula used by the U.S. Coast Guard for monohull powerboats under 20 feet without a capacity plate is simple logic based on physical space and buoyancy.
The U.S. Coast Guard’s traditional 1/15th rule estimates passenger capacity based on available deck space, not actual boat weight or buoyancy.
The number 15 represents an assumed ~15 square feet of deck space per person, a spacing guideline developed decades ago under ideal, calm-water conditions. At the time, this rule also assumed an average person weight of about 150 lbs, evenly distributed across the boat.
Today, modern safety guidance recommends using a higher average weight — approximately 185 lbs per person — along with accounting for gear, fuel, and real-world conditions. As a result, the 1/15th rule should be viewed as a spatial estimate and starting point, not a safe loading limit.