The **beam** of a ship is its width at its widest point. The **maximum beam** (B_{MAX}) is the distance between planes passing through the outer extremeties of the ship, **beam of the hull** (B_{H}) only includes permanently fixed parts of the hull, and **beam at waterline** (B_{WL}) is the maximum width where the hull intersects the surface of the water.^{[1]}

Generally speaking, the wider the beam of a ship (or boat), the more initial stability it has, at the expense of secondary stability in the event of a capsize, where more energy is required to right the vessel from its inverted position. A ship that heels on her beam ends has her deck beams nearly vertical.^{[2]}

Typical length-to-beam ratios for small sailboats are from 2:1 (dinghies to trailerable sailboats around 20 ft or 6 m) to 5:1 (racing sailboats over 30 ft or 10 m).

Large ships have widely varying beam ratios, some as large as 20:1.

Rowing shells designed for flatwater racing may have length to beam ratios as high as 30:1,^{[3]} while a coracle has a ratio of almost 1:1 – it is nearly circular.

The beam of many monohull vessels can be calculated using the following formula:

Where LOA is Length OverAll and all lengths are in feet.

Some examples:

- For a standard 27 ft (8.2 m) yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27 ft monohulls is 10 ft (3.05 m).
- For a Volvo Open 70 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around 18 ft (5.5 m).
- For a 741 ft (226 m) long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around 82 ft (25 m), e.g. Seawaymax.

As catamarans have more than one hull, there is a different beam calculation for this kind of vessel.

BOC stands for Beam On Centerline. This term in typically used in conjunction with LOA (Length overall). The ratio of LOA/BOC is used to estimate the stability of multihull vessels. The lower the ratio the greater the boat's stability.

The BOC for vessels is measured as follows: For a catamaran: the perpendicular distance from the centerline of one hull to the centerline of the other hull, measured at deck level. For a trimaran: the perpendicular distance between the centerline of the main hull and the centerline of either ama, measured at deck level

Other meanings of 'beam' in the nautical context are:

**Beam**– a timber similar in use to a floor joist, which runs from one side of the hull to the other athwartships.**Carlin**– similar to a beam, except running in a fore and aft direction.

The beam of many monohull vessels can be calculated using the following formula: