Boat Persons Capacity Calculator (USCG Length × Width ÷ 15 Rule & Weight)
Estimate how many people a small boat can safely carry with the US Coast Guard length × beam ÷ 15 rule, the matching persons weight at 185 lb per adult, how gear, fuel and a motor eat into the total, and why the capacity plate and calm-water assumption matter.
How it works
USCG rule of thumb (monohulls < 20 ft): persons = floor(length × beam ÷ 15), lengths in feet · persons weight ≈ persons × 150 lb (historic basis); modern adults ≈ 185 lb · people allowed by weight = (maximum capacity − gear − motor) ÷ average person weight
The Coast Guard's quick check for small open boats multiplies length by beam in feet and divides by 15 to get a number of people. It comes from the days of 150-pound occupants and calm water, so it is a ceiling, not a target: heavier crews, coolers, full fuel tanks and a big four-stroke on the transom all take from the same weight budget. Where a boat has a capacity plate, it wins — it was calculated from the boat's actual flotation and stability. This calculator shows both the headcount and the weight arithmetic so you can see which one limits you today.
Worked example
A 16 ft × 6.5 ft aluminium runabout: 16 × 6.5 ÷ 15 = 6.93, so 6 persons by rule (about 900 lb of people at 150 lb each). With modern 185 lb adults that is 4 people; without a plate the estimated total capacity is about 1,215 lb, and after 150 lb of gear and roughly 165 lb of outboard there is about 900 lb for people — again 4 adults in calm water, and 2–3 when it is choppy.
Frequently asked questions
Is the length × width ÷ 15 rule a law?
No — it is the USCG's published estimate for boats without a plate. Federal law requires manufacturers to fit a capacity plate on monohull boats under 20 ft; state laws generally make it an offence to exceed the plate. The plate is calculated from actual displacement and flotation and always takes precedence.
Why does my plate say fewer people than the rule?
Because the plate uses the boat's real weight, flotation and the maximum motor weight rather than a rectangle estimate; deep-V hulls, heavy construction and big motors reduce the persons figure. Since 2011 plates use 185 lb per person.
Does the rule apply to pontoons or inflatables?
Not directly. Pontoon capacity depends on tube volume, and inflatables on buoyancy tube size; their manufacturer plates or CE category labels are the reference. The adjusted factors here are only rough guides.
Related calculators
Boating figures are planning estimates based on published rules of thumb and standards (USCG, ABYC, manufacturer tables). Your boat's capacity plate, builder documentation and local regulations govern; conditions at sea vary.