Spell out numbers up to and including nine; use numerals for numbers larger than nine, including ordinals (Eighth Conference on Severe Local Storms, 12th Symposium). If numbers in the same category both greater than and less than nine appear within the same context (e.g., a paragraph), follow The Chicago Manual of Style and use numerals for both: "each dropwindsonde is about 9 inches (23 centimeters) long and 3.5 inches (9 centimeters) in diameter." (Prior to fall 2002, our style was to spell out numbers up to and including ten.) [01-03]
Numerals are always used with abbreviated units of measure: 25 km, 1 m, etc. If the unit of measure is not abbreviated, rule 1 above applies: 12 months, six kilometers, etc.
Use numerals with very large dollar amounts and large fractions: $3 million, 6.28 million dung beetles. Spell out whole numbers from one to nine, except when you wish to convey a higher level of precision (editor's choice): about two million refugees, but they counted 8 million species. See The Chicago Manual of Style.
Use an en dash within a number range: 6–12 months, but when preceded by "from," insert "to": from 6 to 12 months. (Also see: en dash)
Hyphenate numbers and units in adjectival phrases: six-month term, 20-year appointment, 9-km length, etc.
Insert a zero before decimal points (in numbers less than 1) except in statistical correlations, and even there if it would look peculiar not to.
To make numbers plural, add an s (with no apostrophe): 1940s, 16s, ones, fives, 10s, etc.
Use commas in four-digit and larger numbers except those expressing wavelengths: 3,124 years but 4120 A. Four-digit page numbers have no comma: pg. 5769; five digits or more take a comma for legibility: pg. 11,275. [revised 2/00—CR]
Use numerals for numbers used as nouns: a factor of 2. Zero is an exception: value of zero.
Avoid negative exponents whenever possible: 1.7 X l0-5 photons cm-2 s-1 keV-1 becomes 1.7 X l0-5 photons/(cm2 s keV). Note, however, that wave numbers (the reciprocal of wavelength) are expressed with negative exponents.