Boat Capacity & Load Calculator
Maximum persons from length and beam, plus a load check against the plate — people, gear and motor — with what the plate does and does not cover.
The capacity plate on your boat always wins. This formula is for hulls under 20 feet that have lost their plate or never carried one. Where a plate exists, the manufacturer's figure accounts for hull shape in a way length times beam cannot.
Leave the plate limit at zero if you do not have one — the person count still applies.
Where the formula comes from
Length times beam divided by 15, both in feet, rounded down. That is the US Coast Guard rule for monohulls under 20 feet, and it is deliberately crude — it is a floor for boats with no better information, not a target to fill.
Note the assumption buried in it: people are counted at 150 pounds each. That average has not moved with the population, so a boat carrying its "maximum" number of adults may already be over on weight while still being legal on the count.
What the numbers do not cover
- The capacity plate always wins. The formula is for boats under 20 feet that have lost their plate or never had one, and a manufacturer's figure accounts for hull shape in a way length times beam cannot.
- Capacity assumes good conditions. A number that is comfortable on a flat lake is not a number to carry into a chop, and no plate anywhere accounts for weather.
- People are counted at 150 lb each in the US formula. That average has not moved with the population, so a boat carrying its "maximum" adults may already be over on weight.
- Weight sits where you put it. The same total load is far worse concentrated at the transom or on one side, and small boats are lost to poor distribution more often than to gross overloading.
How to use
- Read your capacity plate if the boat has one.
- Enter length and beam for boats without a plate.
- Total the weight of people, gear and motor.
- Stay well under the limit rather than at it.
Frequently asked questions
How is the persons capacity calculated?
For small outboard boats without a plate, a common rule is length times beam in feet divided by 15, rounded down. It is a rough guide only — the capacity plate, where one exists, is the authoritative figure and is derived from actual stability testing.
Does the plate include the motor?
The maximum weight capacity typically covers persons, gear and motor together, and it is easy to overlook how much a modern outboard and a full fuel tank weigh. Loading to the persons limit and then adding a heavy motor and full tanks can exceed the weight limit while appearing to comply.
Is the capacity limit a legal requirement?
In many jurisdictions, exceeding it is an offence as well as unsafe, and it can affect insurance. The plate is placed under regulation for boats of certain sizes, and its absence on an older or larger boat does not mean there is no limit — it means the limit is not printed.
Does the limit assume good conditions?
Yes, and this is the part that matters. Capacity figures assume calm water and reasonable weight distribution. In chop, in wind, or with everyone on one side, a boat at its rated capacity can be genuinely unsafe. Loading well under the limit is normal seamanship rather than caution.
Why does weight distribution matter so much?
Because stability depends on where the weight sits, not merely how much there is. Weight high up or concentrated at one end reduces stability sharply, and passengers moving suddenly to one side is a recognised cause of small-boat capsizes.
What about children and gear?
Capacity plates generally assume an average adult weight that no longer reflects reality well, so counting heads can understate the load. Weighing the actual people and gear against the weight limit is more reliable than counting seats, and coolers, batteries and water weigh more than people expect.
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