The Maximum safe load and the persons capacity that a boat can carry is based on the displacement weight of a boat. What is displacement weight? That is essentially the amount of weight that it would take to sink your boat. There are several ways to find this out.

1. You can calculate the volume of water displaced (hence displacement) when the boat is sunk to the point where water starts to come in, also called the static float plane. Multiply this volume times 62.4. (The weight of one cubic foot of fresh water.): or, (Click image for full size.) 2. Or put weight in the boat until water starts to come in. This sounds simple but is difficult for the average boat builder because it requires a lot of weight. The amount of weight can be considerable. On a larger boat it can be 10,000 lb. or more. The amount of weight it takes to sink the boat is the displacement weight.

 If you want to do it this way hire a test lab or contact the Coast Guard. See the flotation page for information about testing.

3. Or with smaller boats, fill the boat with water using a bucket of a known amount in gallons (or Liters). Do Not do this with the boat out of the water. The weight of the water will break the boat. First float it with some depth under it. Fill it until the level of the water in the boat and outside the boat is equal, that is, when water starts to flow in and out of the boat. Multiply the number of gallons times 8. One liter of fresh water weighs about 1 Kg. That gives you the displacement weight.

Safe Loading - Testing For Capacity

How to test a boat to determine safe load and number of passengers.

The slide show depicts how a boat is tested for maximum weight capacity. These were taken at the US Coast Guard's testing facility in Solomon's Island, Maryland, during a training session for Coast Guard employees, the contractor that performs the testing, and the contractor that conducts visits to boat manufacturers.

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Inboard, Stern-Drive, and Electric Boats (not electric outboards) Boats under 20 feet.

NOTE: Electric Propulsion boats include weight of motors, batteries, controllers: the whole propulsion package as machinery weight or boat weight.)

Maximum Weight Capacity=(displacement weight-boat weight-4(machinery weight))/5 = W

or:

Maximum Weight Capacity = (Maximum Displacement - boat weight)/7 = W

Maximum Persons Capacity = W; or for boats with W less than 550 use the test method.

Then measure the following on your boat: (Click image for full size) Cockpit area. 40% reference area. Passenger area 70% reference areas. Two foot reference areas fore and aft. Test Method: float the boat in calm water with all the normal gear aboard, that is, engines, batteries, controls, etc.

Table converting inches to decimal feet

  1 in = 0.083 ft
  2 in = 0.16 ft
  3 in = 0.25 ft
  4 in = 0.33 ft
  5 in = 0.42 ft
  6 in = 0.50 ft
  7 in = 0.58 ft
  8 in = 0.67 ft
  9 in = 0.75 ft
 10 in = 0.83 ft
 11 in = 0.92 ft

Add weights along the outboard side of the passenger area . The weight should be at seat height and distributed equally for and aft. Add weights until water is about to come over the gunwale. Stop. Add up the weight.

Maximum Persons Capacity = Total of Weights/0.6

Maximum Persons (in people) = (Maximum Persons Weight + 32)/141. Round up or down.

A bit of advice. Maximum persons capacity doesn't have to be the maximum amount, it can be less. Smart boat builders down rate the maximum weight and persons capacity to cover liability and all that other heavy junk people carry on to their boat like coolers full of beer, extra gas, the spare fish finder they just have to have, the ski-boards and slalom skis, etc., etc. REMEMBER! The boat operator will exceed whatever you put on the label and then blame you if something goes wrong! Be conservative. Give yourself some room.

Outboard Boats under 20 feet rated for greater than 2 HP.

Maximum Weight Capacity = W- boat weight /5

Maximum Persons Capacity = Maximum Weight Capacity - Col 9 Table 4
NOTE: Table 4 is for gasoline powered outboards. For Electric Outboards use the actual published weight of the outboard.

Outboard boats under 20 feet, 2 HP or less.

Maximum Weight Capacity = (W- boat weight) x 3/10

Maximum Persons Capacity = (Maximum Weight Capacity - 30) x  0.90

Maximum Persons (in people) = (Maximum Persons Weight + 32)/141   Round up or down.

Manually propelled boats.

Maximum Weight Capacity = (W- boat weight) x 3/10

Maximum Persons Capacity = Maximum Weight Capacity x 0.90

Maximum Persons (in people) = (Maximum Persons Weight + 32)/141  Round up or down.

The same warning applies to Inboard boats. Be conservative. Error on the side of safety.

Since I first wrote this, software and phone apps have become available that can help you determine the displacement of the boat. These direct you to take pictures of the boat from all sides and then the app or software converts that to a 3D image. The 3D image is used to calculate the volume and displacement. The only one I am familiar with is polycam, https://poly.cam/ (This is not an endorsement. It is only an example of what is available.)

Also if you are fairly handy with computer assisted design software, there are packages available that can do all of the calculations, called hydrostatics by naval architects. The most well known of these is Delftship, https://www.delftship.net/ (I have no financial interest in Delftship. I offer it only as an example) available free for the limited edition. There are more sophisticated packages available for a price. I have used delftship, and it is quite a powerful package, but you do need to know what you are doing. There are forums, such as https://www.boatdesign.net/ that can assist you.

For Boat Owners:  Some common Myths about boat load capacity.

Capacity

Belief:  I have both a capacity label and a label showing the number of seats. If the seating label has more seats than people on the capacity label, then I can use the seating label as the capacity.

Not True:  The seating label is placed on the boat voluntarily by boat manufacturers to show you where it is safe to sit.  This grew out of boat manufacturers trying to find a way to show boat operators and passengers where it is safe to sit to prevent injuries and deaths.  Even though it is common sense that it is unsafe to sit on a gunwale, or transom, or the bow, and especially on pedestal seats when the boat is underway, some people just don't get the word. Boat operators often fail to follow safe operating basics, such as sitting in designated seats when the craft is on plane, maintaining a balanced load or giving themselves an unobstructed view from the helm.  Many boats have casting platforms, storage lockers and other surfaces which are inappropriate for seating when a boat is on plane.  So the American Boat and Yacht Council and the National Marine Manufacturers Association developed a label showing where it is safe to sit. It is simply an information label.

Recreational boat manufacturers are required to put a capacity label on mono-hull boats under 20 feet in length. The number of persons on the capacity label is the number you should use. You will see a similar label on many other types of boats, including canoes, kayaks, pontoon boats and inflatables.  The manufacturers voluntarily (in the USA) put these on other boats because it is good business and safety sense to do so. In Canada and Europe the label is required on all recreational boats (with a few exceptions).In the USA the label is a manufacturer requirement. The boat owner does not have any responsibility under Federal law for this label, not even for complying with the values on the label. (they do in Canada)  However, most states have passed laws making it illegal in the particular state to exceed the capacities on the label. So if you live in one of those states you can be cited by state or local law enforcement for exceeding the capacity on the label. Also your insurance company may take a dim view of exceeding the capacities.  So, use the Capacity Label as a guide to how many passengers you can carry, and the seat label as a guide to where they can safely sit.

Belief:  I have an old boat with no capacity label, but I found a formula in a boating book that says if I multiply length (L) times the width of the boat (B for Beam) and divide by 15 that will tell me how many people I can carry. 

Maybe:  This formula, L X B/15, has been around a long, long, time.  I have traced it as far back as the 1940’s.  In the boating world, that is ancient. It has shown up in a lot of well respected boating publications, books, and even in the US Coast Guard Boatbuilders Handbook. It is not the formula that boat manufacturers and builders are required to use to determine persons capacity for recreational boats, or for commercial passenger carrying boats. The formula is a rule of thumb. It is only meant to be used as a rough estimate. It is in the handbook only as a way to estimate the number of passengers.  It is also only intended for use with small mono-hull boats, usually outboard powered but it has been used for small inboards.  It should never be used for boats more than 25 or 26 feet in length.  It is simply not applicable to larger boats. It also does not apply to canoes, kayaks, multihulls, inflatables, or pontoon boats.

Belief:  The US Coast Guard uses 160 pounds per person to rate the capacity of recreational boats and/or commercial boats.  I hear this one everywhere. It was recently stated in a BOATUS article. 

Not True: Here is an e-mail I sent to BoatUS concerning this.

"The Coast Guard does not and has never used 160 pounds per person for capacity ratings on recreational boats.  This is a common boating myth and where it got started is beyond me.   But since I spent 25 years working in the Coast Guard Office of Boating Safety I think I can explain how capacity is determined.

It starts with displacement weight. Displacement weight is the amount of weight a boat would displace if you weighed the hull down to the point where water would start to come in. Essentially that is the weight it would take to sink the boat.  That amount is divided by five for outboard boats, and by seven for inboards. The result is the Maximum Weight Capacity.  That is the amount of weight you can safely put in the boat including motor gear, batteries, controls and people.  To find the persons capacity you then subtract the outboard motor weight, batteries, full fuel tank.  But a manufacturer must use the outboard weights in the Coast Guard's Table, or the ABYC table for outboard weights. The result is persons weight in pounds. To find the number of persons those pounds represent, the Coast Guard uses a formula: persons weight plus 32 divided by 141.  The result is rounded to the nearest person. 

Also the boat manufacturer does not have to use this maximum number. They cannot exceed the numbers for maximum weight capacity, maximum persons capacity and persons but they can use a smaller number, and many manufacturers do downrate.  The amount of flotation that goes into the boat is based on; the hull weight, the persons weight and the outboard motor weight.

For inboards a similar exercise is used to determine  maximum weight capacity, maximum persons capacity and persons. But the engine is not treated separately. It is considered part of hull weight.

Perhaps the confusion comes from commercial vessels.  On the commercial side of the Coast Guard they use a persons weight to determine how much weight to use when performing stability tests. Here s a quote from a paper on this on my web site entitled How Much Of A Load Is Too Much?:

On a commercial vessel, the number of passengers is estimated based on the lesser of the following criteria:

1.  Length of rail: one passenger for each 30 inches of rail at the sides and stern., or,

2.  The deck area; one passenger per 10 square feet of deck area, excluding spaces listed in 46 CFR 176.113, which include, among other areas, concession stands, toilets, lifesaving gear storage spaces, required aisle area,  or

3. The fixed seating areas, or fixed seating; one passenger for each 18 inches of fixed seating width.

The stability of the vessel is then determined using the number of passengers allowed based on the initial determination. An SST (simplified stability test) is conducted, based on the Coast Guard criteria of 140 pounds per person.  If the boat does not pass the stability test then the number of passengers is based on the weight of passengers that would pass the test at 140 lbs per passenger. The Ethan Allen investigation has determined that this weight is too low, and should be at least 174 pounds.  The Coast Guard has raised it to 184"

See HOTTOPICS; How Much Of A Load Is too Much?

However, I was inaccurate.  The figure 140 pounds per person was used for boats with a combination of adults and children,  but if all the passengers were adults, then 160