See also NUMBERS.
Form plural with s, no apostrophe.
In general use figures for the following:


cardinal and ordinal numbers larger than nine, ninth
ages of persons, animals, and things above nine
calibers: .50caliber machine gun, 40mm cannon
centuries: the 1800s
dates
decimals
Egyptian dynasties: 1st dynasty, 12th dynasty
fractions (See FRACTIONS.)
lens apertures: f/16
latitude and longitude: 72° 54' 23" N, 165° 53' W
page numbers
percentages; but one percent
sessions of Congress above the Ninth
sums of money that are not whole units or that are above units of nine:
$1.21, $11 (not $11.00); but eight dollars. (See MONEY.)
temperatures
times of day with a.m. and p.m.

Use Roman numerals for rulers, popes, Olympiads, personal names, ships,
and a few other special uses. For formation, see Number Table in Webster's. 
