Technical writing can often include a lot of numbers, and it can be hard to keep track of how they should all be styled. Sometimes they are spelled out and sometimes numerals are used – but how do you know what to do in each situation?
Different style guides will have different guidelines, but generally you will have a rule to spell out zero to __ and use numerals for anything above. The Chicago Manual of Style, for example, says to spell out zero through one hundred (plus certain round multiples of these) in nontechnical contexts, whereas with other style guides (for example, The Associated Press Stylebook), the rule is to spell out numbers below 10 and use numerals for 10 and above. The below 10 option is also generally preferred for more technical or scientific contexts.
As always, there will be exceptions and special cases in addition to the general rule above. These will vary by style guide, but below are a few examples of decisions you can make:
- In nontechnical material, the treatment for physical quantities can usually follow the general rule. For more technical material, you may decide to use numerals more frequently, especially if you deal with a lot of numbers and doing so will provide clarity for your readers.
- If you have a mixture of numerals and spelled out numbers for the same category of items within a series, some style guides will advise that you make them consistent (use all numerals), while others will just leave the mixture. Sometimes the decision to treat all numbers of the same category (for example, all measurements or all ages) within the same document can be made as well, particularly if these numbers appear frequently.
- If the number is part of a brand or other official name, follow the respective organization’s style.
- Spell out numbers in the following situations:
- When the number begins a sentence. In cases where this might be awkward and you would normally use a numeral, such as if the sentence starts with a year, it may be better to reword the sentence.
- With simple/common fractions (e.g., two-thirds, three-fourths, one quarter, half).
- With time when used with o’clock.
- Use numerals in the following situations:
- In dates and years.
- In street addresses (unless spelled out as a street name, e.g., 48 Second Street).
- With units of measure if using the abbreviated form or symbol for the unit.
- With amounts of money when using a symbol ($3.00, $0.97, $124). You don’t have to include the decimal point with round amounts, but you should include it for consistency if the round amount is mixed with other amounts that require it.
- With percentages, followed by either percent or %.
- With decimals or with whole numbers that include fractions (e.g., 3.14, 5¾).
- When referring to pages, chapters, steps, figures, sections, etc.
- For exact times of day (e.g., 3:10 p.m.)
As you can see, there are a wide variety of situations where numbers are used where you will need to decide whether to spell them out or use numerals. Keep some of these examples in mind when you examine your particular situation and make the decision that is most suitable for your own documentation – just keep track of those decisions and apply them consistently for the best results!
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