Measuring a ship is difficult. Naval Architects and Government taxing authorities have experts on their payroll that make a career out of measuring ships. What at first seems to be fairly simple is really complex. You would think that you could measure the length, width, and depth, and then weigh the ship (if you could find a scale big enough) and that would be it. No more data would be necessary.
Unfortunately, it is much more complicated than that. We use different statistics for different purposes. Taxing authorities are concerned with the internal volume of the ship, especially that part of the volume that is used to carry cargo and make money. They use what we call the Gross and Net tonnage. These numbers have nothing to do with the weight of the vessel. A volume of 40 cubic feet is considered to be a ton. A vessel pays taxes on its Net Tonnage, which is the internal volume of cargo spaces. Gross tonnage is used in filling out most forms, and is the most important tonnage for legal purposes. It is the internal volume of the entire ship, including engine, crew, and storage spaces which don't count in the Net tonnage.

The actual weight of a ship is called Displacement Tonnage. The tons used are Long Tons of 2240 lbs. It is fairly close to a Metric Ton of 2204 lbs. Short tons of 2000 lbs. are not used. When the ship is built, but has on board no fuel, water, stores, crew, or cargo, it is at it's "Light Displacement". A ship, when it floats, displaces water. This is called Archemedes' Principle. It has to displace a volume of water equal to it's weight in order to float. When a ship is loaded down to it's load line (Plimsoll Mark) it is at it's "load displacement", or maximum permitted weight. The difference between the Load Displacement and the Light Displacement is the Deadweight Tonnage (d.w.t.), or actual lifting capacity of the vessel. Of course not all of the Deadweight capacity of a ship can be used for cargo. You must have fuel, water, and stores. Ships float in waters of different densities. A cubic foot of sea water weighs more than a cubic foot of fresh water. Therefore a ship will displace more cubic feet of fresh water than of salt water if its weight remains the same. That is to say, it will settle deeper into fresh water and have a deeper draft when it sails from salty ocean water to fresh harbor water. The difference is not too great, about 3.5%, but it means about 8" (20 cm) increase in the ship's draft. When loading a ship you must take into account the density of the water around the ship at the time you read the draft. You use a hydrometer for checking the density, similar to that used in checking an auto battery.

Now the difficult part starts. A ship's weight is constantly changing! The ship's officers must constantly monitor the ship's weight and center of gravity. If you load too much cargo, you float too deeply in the water and exceed your permitted draft (load line). In many places you can also be too light, and not have enough draft to safely maneuver the ship. The propeller and rudder, as well as the bow have to be submerged for the ship to handle well. If the center of gravity moves too far forward you go "down by the head" which usually means excessive fuel consumption and poorly functioning propeller and rudder.

If the center of gravity moves too far aft, you get excessive "trim by the stern" and again waste fuel, since the ship is always "pushing up hill". The center of gravity also moves up and down. If it gets too high, the ship can capsize, or turn upside down. If it gets too low, you have "excessive GM" and are too stiff. The ship rocks and rolls because the natural roll of the ship is so short it coincides with the wave period.

Why does the weight and center of gravity change? Obviously it changes as we load and unload cargo. Before we leave the dock, we must compute the center of gravity, which gives us an approximation of our draft forward and aft. This computed draft must match fairly closely the actual observed draft. We must then compute the center of gravity when we arrive at our next port. To do this, we must calculate how much fuel we will burn, and what tanks the fuel will come from. Most fuel tanks are down low in the ship, so as we burn fuel the center of gravity will rise. We can offset this rise (and loss of stability) by ballasting, but nobody wants to waste fuel by carrying around sea water ballast. Also many ports of the world no longer want you to pump out your ballast water in their port. The center of gravity also changes as we make and use fresh water and stores, but usually those changes are small in relation to changes in the cargo and fuel. Anyone who has ever piloted an aircraft also knows the importance of knowing your center of gravity. Aviation and Navigation share many things in common.

The length of a ship is also a number that can be vague. All exterior dimensions can be listed as "over all" or "molded". Molded is used on the ship's plans, and is what the naval architects designs show. They do not include the width of the shell plating or other features that may stick out from the hull. Length between perpendiculars is also a number used. The Forward and After Perpendicular are usually located at the front and back of the ship's keel. The length of the bow and stern are not counted in this length, which again is a number used by taxing authorities. Length Overall is the most important number. It is used in paying wharfage fees, and is the actual length of the ship.

As you can see, when someone asks "How big is your ship?" I am unable to give them a very precise answer.