The foot mark is a prime
The proper symbol for feet is the prime ′ (U+2032), a short slanted stroke. Put it right after the number, with no space: 10′, 6′ 2″. One foot equals 12 inches, or exactly 30.48 cm.
The prime has other jobs too. In angles and coordinates it means minutes of arc, so 51° 30′ is fifty-one and a half degrees. In math it marks a derivative or a related value: f′(x), A′. Inches and seconds of arc use the double prime ″.
Prime vs apostrophe
Most keyboards have no prime key, so people type the straight apostrophe ' instead. That is acceptable in plain text, but smart punctuation in Word, Google Docs and on iPhone often turns it into a curly ’, the mark used in words like it’s. A height written 6’ 2” looks off to anyone who knows typography, and a curly opening ‘ in front of a number is plainly wrong.
Two more look-alikes to avoid: ʹ (modifier letter prime) is used to transliterate Cyrillic, and ´ is an acute accent. They look close to ′ but are separate characters, so search will not treat them as feet. For quote marks themselves, see quotation mark symbols.
How to type the feet symbol
In Word, type 2032 and press Alt + X for ′ (or 2033 for ″). If Word curls an apostrophe you typed, press Ctrl + Z immediately to get the straight ' back. On a Mac, search for "prime" in the Character Viewer (Control + Command + Space).
In HTML, use ′ for ′ and ″ for ″. In LaTeX math mode, f' or ^\prime gives a prime. On iPhone, turning off Smart Punctuation (Settings > General > Keyboard) keeps ' straight; for the true prime, copy ′ from this page.
Writing a height for a bio or dating profile? Copy one of the ready-made heights above so both marks are right. To mark angles, see the degree symbol.