The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship (or boat), the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position. Typical length-to-beam ratios for small sailboats are from 2:1 (dinghies to trailerable sailboats around ) to 5:1 (racing sailboats over ). 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 , while a coracle has a ratio of almost 1:1 - it is nearly circular.

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

Beam = LOA (Length Overall) in feet to the power of 2/3 + 1

Some examples

- For a standard 27' (8.23m) yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27' monohulls is 10' (3.05m).

- For a Volvo Open 70 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around 18' (5.5m).

- For a 741' (226m) long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around 82' (25m) e.g. Seawaymax.