When to spell out numbers

Many writers have difficulty figuring out when to write numbers in words or figures. There are some general rules but these are not applicable in all contexts. The following guidelines should, nevertheless, help.
The numbers of Kings and Queens should be written in Roman characters.

• Examples: Elizabeth II, Louis XIV

Ordinal numbers up to twelfth should be written in words except in dates.

• He finished second. (NOT He finished 2nd.)
• Who came first? (NOT Who came 1st?)

• He was born on 3rd May. OR He was born on May 3rd. (BUT Not normally He was born on third May.)

Cardinal numbers up to 12 should be written in words, except when telling the time.

• We need three chairs and one table.
• He has three sisters.
• The train leaves at 5 pm.

Cardinal and ordinal numbers above twelve and twelfth should be written in either figures or words as seems in each case more convenient.

• There were 50 students in the class.

Technical writing, however, is different. In technical writing any numbers used with measurements of time or distance should be expressed in figures. Examples are: 5 years, 2 months, 6 inches etc.
There is some difficulty in defining the rules, but generally any number that represents a key value is  written in figures. For example in the sentence, ‘This chipset is based on the 32nm technology’ the numeral is in order because the number 32 has a key value in the context. When the number is not very ‘important’ it can be written in words.
Even in technical writing, sentences shouldn’t begin with numerals. In such cases write the number out in words or rephrase the sentence so that the number doesn’t begin the sentence.
According to the Chicago Manual of Style, whole numbers from one through one hundred, round numbers and any number beginning a sentence should be spelled out. For other numbers, numerals are used.

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