Calculating Your Boat's Capacity
On boats less than 20 feet in length with no capacity plate, use the following rule of thumb to calculate the number of persons (weighing 150 lbs. each, on average) the boat can carry safely in good weather conditions.
For example, for a boat 18 feet long by 6 feet wide, the number of persons is 18 times 6 (or 108) divided by 15, which equals seven 150-lb. persons (or a total person weight of 7 x 150, or 1050 lbs.).