Spell out numbers one through nine. Use figures for numbers 10 and larger. The major exceptions are numbers representing units of measure, time and money, which are always expressed in figures (also called cardinal numbers). Other exceptions include:
Numbers at the beginning of sentences should be spelled out.
All scores and ratios should be written in figures.
Numbers smaller than 100 which designate political or military divisions should be spelled out: Seventh Ward, Second Congressional District.
Percents: see item 3.
Ordinal numbers designate the place of a number in an ordered sequence (first, second, third). For ordinal numbers, spell out first through ninth when they indicate sequence in time or location: first base, the First Amendment, he was first in line. Starting with 10th, use figures. Although the day of the month is actually an ordinal (and so pronounced in speaking), write it as a cardinal number: April 18, not April 18th.
In text, percents should be written out in figures with the word percent, even for figures under 10: 30 percent, 8 percent. Use decimals instead of fractions: 6.5 percent, 2.4 percent. For amounts less than 1 percent, precede the decimal point with a zero: 0.6 percent. Repeat percent with each individual figure: He said between 10 percent and 30 percent of the electorate will vote.
The plural of figures is the addition of an "s": 1970s.
In narrative copy, spell out a fraction when it stands alone: one half. Figures may be used in tabular material. Use figures for precise amounts larger than one, converting to decimals whenever practical: 13 and 3/4 or 13.75.
For whole dollar amounts, do not use ciphers after the whole number: $10, not $10.00. Amounts of money less than $1 should be written in figures with the word cents: 60 cents, 99 cents.