When do I spell a number versus write the numerical form of the number?
This is the type of question that doesn't work well in email, since a follow-up question on my part is a much lengthier process than it is when we have a face-to-face conversation.
I am going to assume you are asking this because you are writing a paper for school, and therefore are interested in a writing style, such as APA or MLA. I will answer this question based on this assumption. If you are using a different style, then you will need to let me know.
Numbers in APA (here follows an excerpt, see pp. 111-112 of the Publication Manual of the American Psychological Association, 6th ed. for more details).
4.31 Numbers expressed in Numerals
a. numbers 10 and above
12 cm wide, the remaining 10%, 25 years old, 10th-grade students, etc.
b. numbers in the abstract of a paper or in a graphical display within a paper.
c. numbers that immediately precede a unit of measurement
a 5-mg dose, with 10.54 cm of
d. numbers that represent statistical or mathematical functions, fractional or decimal quantity, percentages, ratios, and percentiles and quartiles
multiplied by 5, 3 times as many ...
e. numbers that represent time, dates, ages, scores and points on a scale, exact sums of money, and numerals as numerals.
1hr 34 min, at 12:30 a.m., 2-year-olds, scored 4 on a 7-point scale - Exception: Use words for approximations of numbers, days, months, and years (e.g., about three years ago).
f. numbers that denote a specific place in a numbered series, parts of a book and tables, and each number in a list of four or more numbers.
Grade 8 (but eighth grade ...), Table 3, row 5
4.32 Numbers Expressed in Words
Use words to express
a. any number that begins a sentence, title, or text heading. (Whenever possible, reword the sentence to avoid beginning with a number.)
Forty-eight percent of the sample showed an increase; 2% showed no change., Twelve students improved, and 12 students did not improve
b. common fractions
one fifth of the class, two thirds majority
c). universally accepted usage
the Twelve Apostles, Five Pillars of Islam
[There is some more, but I hope this is enough ... ].
Numbers in MLA (excerpt, see pp. 81-85 of the MLA Handbook for Writers of Research Papers, 7th ed. for greater detail).
3.5.2 Use of Words or Numerals
If you are writing about literature or another subject that involves infrequent use of numbers, you may spell out numbers written in one or two words, and represent other numbers by numerals (one, thirty-six, ninety-nine, one hundred, fifteen hundred, two thousand, three million, but 2 1/2, 101, 137, 1,275). To form the plural of a spelled-out number, treat the word like an ordinary noun (sixes, sevens).
If your project is one that calls for frequent use of numbers - say, a paper on a scientific subject or a study of statistical findings - use numerals for all numbers that precede technical units of measurement (16 amperes, 5 milliliters). In such a project, also use numerals for numbers that are presented together and that refer to similar things, such as in comparisons or reports or experimental data. Spell out other numbers if they can be written in one or two words. In the following example of statistical writing, neither "ten years" nor "six-state region" is presented with related figures, so the numbers are spelled out, unlike the other numbers in the sentence.
In the ten years covered by the study, the number or participating institutions in the United States doubled,
reaching 90, and membership in the six-state region rose from 4 to 5.
But do not begin a sentence with a numeral.
Two thousand four was an election year in the United States.
Except at the beginning of a sentence, always use numerals in the following instances:
WITH ABBREVIATIONS AND SYMBOLS
6 lbs., 4:20 p.m., 3%, 8 KB, $9, 2"
4401 13TH Avenue
1 April, 2007
April 1, 2007
IN DECIMAL FRACTIONS
page 7, year 3 of the study
For large numbers, you may use a combination of numerals and words.
Express related numbers in the same style.
only 5 of the 250 delegates, exactly 3 automobiles and 129 trucks, from 1 billion to 1.2 billion
[There is more, but hopefully this is enough].