Ship measurements are used to determine a ship's overall size and capacity. While these measurements are typically provided in the standard length, width, and depth measurements of any other three-dimensional object, there are specific reference points used in calculating ship measurements, as well as specific acronyms used to denote differences in the reference points being used. Knowing these can help one make sense of ship measurements.
A Length Overall ship measurement (LOA) is provided in feet and inches. It is measured from the extreme furthest end of the ship's bow to the extreme furthest end of its stern. While this provides a suitable geometric measurement for the ship, it is not quite as useful at indicating the amount of usable space on the vessel, since the prow and other parts of the ship which sit further ahead of the furthest point of the deck are included in this measurement.
Length Between Perpendiculars ship measurements (LBP) offer better estimates of ships' carrying capacities. These are measured in feet and inches, and given from the forward point of the stern to after the sternpost. They typically exclude the unusable portions of the deck, providing more accurate readings for the useful space on the vessel.
Length on Load Waterline (LWL) measures the length of the ship at the waterline. In other words, it measures the length of the ship that is actually sitting at the water's surface. This measurement is typically only used in calculating the potential speed and water resistance of a ship.
The width of a ship is typically measured from the outermost point on one side of the ship to the outermost point of the other. This is measured at the point where the ship is "fattest," providing the widest possible point on the entire vessel. Like the other measurements, it is provided in feet and inches.
Depth is measured from the lowest possible point on the hull to any indicated reference point on the side of the ship. This is typically measured to one of the draft points on a ship's hull, which are markings spaced six inches apart. Draft marks are placed to indicate the distance from the base of the keel to the waterline, allowing the crew to know how far beneath the water the hull of the ship currently is.