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.

Slide Show of a boat being tested for Maximum Weight Capacity.

3. Or with smaller boats fill the boat with water using a bucket of a known amount. 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. That gives you the displacement weight.

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Inboard and Stern-Drive Boats under 20 feet.

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

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.

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