Reader Pam points out a significant difference between technical writing and non-technical writing:
On your 10 rules for numbers, rule #2 doesn’t tell the whole story, especially for technical writers. Any numbers used with measurements (time, distance) must be expressed as figures in technical writing (8 months, 6-inch nails, 9 p.m.)
With any kind of writing, context determines usage, and a style guide appropriate to the genre must be the writer’s final authority.
Writing online, David A. McMurrey, author of Processes in Technical Writing (Macmillan 1988), has this to say about the use of numerals in technical writing:
The main hurdle to overcome is to learn that in technical contexts, we use numerals in text, even ones below 10. In other words, we break the rules that are taught in regular writing courses and that are used in normal publishing and copyediting practice. That’s because in the technical and scientific context, we are vitally interested in numbers, statistical data, even if it’s a 2 or 5 or—yes—even a 0.
He acknowledges the difficulty in defining the rules, and distinguishes between “important” and “unimportant” numbers.
You should use numerals, not words, when the number is a key value, an exact measurement value, or both. For example, in the sentence “Our computer backup system uses 4 mm tape” the numeral is in order.
He gives these examples in which the word is preferable to the numeral:
There are four key elements that define a desktop publishing system.
There are six data types in the C programming language.”
Like the general writer, the technical writer is advised against beginning a sentence with a numeral:
write the number out or, better yet, rephrase the sentence so that it doesn’t begin the sentence.
McMurrey concludes his rules about the writing of numerals with a reminder of the importance of context:
Apply these rules in specifically technical, scientific contexts only. Be sensitive to what the standard practices are in the context in which you are writing.
Recommendations to spell or not to spell a number differ from style book to style book. Many, for example, advise spelling numbers 1-10 and using numerals for eleven and up. I don’t follow that rule because I don’t like the way 11 looks in a sentence.
Here’s the Chicago Manual of Style’s General Rule about the use of numerals:
In nontechnical contexts, the following are spelled out: whole numbers from one through one hundred, round numbers, and any number beginning a sentence. For other numbers, numerals are used.
The CMOS then goes on to treat the numerous exceptions and special cases at length.
Bottom line: When it comes to writing numbers as numerals or as words, consider context, and equip yourself with an appropriate style book for the work at hand.