A square fathom is defined as the area of a square
with sides one fathom in length.

The definition of a fathom is as follows:

unit of length equal to six feet used to measure depth of water

››Metric conversions and more

ConvertUnits.com provides an online
conversion calculator for all types of measurement units.
You can find metric conversion tables for SI units, as well
as English units, currency, and other data. Type in unit
symbols, abbreviations, or full names for units of length,
area, mass, pressure, and other types. Examples include mm,
inch, 100 kg, US fluid ounce, 6'3", 10 stone 4, cubic cm,
metres squared, grams, moles, feet per second, and many more!