[ADRTCK]

The international standard symbol for a foot is ft (see ISO 31-1, Annex A). In some cases, the foot is denoted by a prime, which is often approximated by an apostrophe, and the inch by a double prime; for example, 2 feet 4 inches is sometimes denoted as 2′−4″, 2′ 4″ or 2′4″.

Källa: http://en.wikipedia.org/wiki/Foot_%28unit%29

Har inte riktigt tid att detaljstudera artikeln, men Wiki verkar i alla fall ange fot'tum" som standard.