An ell, when used as a unit of length, is usually 45 inches, i.e. 1.143 m (for the international inch). It is now obsolete and was mainly used in the tailoring business, and is derived from the length of the arm from the shoulder to the wrist.

In English this unit has mostly fallen out of use, as do others based on the human arm: digit (1/60 ell), finger (7/360 ell), palm (1/15 ell) hand (4/45 ell), shaftment (2/15 ell), span (1/5 ell) and cubit (2/5 ell). The exact length was never defined in English law.

Several different national forms exist, with different lengths, including the Scottish ell (approximately 37 inches), and the Flemish ell (approximately 27 inches).

Sometimes an ell is used as an alias for the cubit.