When writing a scientific text, you can hardly avoid using numerals; and sometimes one has to use them even in writing on subjects less concerned with numbers – for example, literature or linguistics. Generally people just use them without thinking about the details – and many of them would probably be very much surprised to find out that figures in writing are used in accordance with a very definite set of rules. For you not to make any mistakes in this field, there are a few most important dos and don’ts about figures.
First of all, single-digit numbers are generally spelled out. Starting with 10 you should use numerals. However, there are cases when this rule is overruled by others, more important ones. For example, you have to choose whether to use words or numbers for a particular category and keep doing it throughout your paper, e.g., it would be wrong to say I saw five birds on the left and 20 on the right. Either use only numerals or only words when you mention birds, but don’t intermit them.
Fractions are a little bit trickier. Simple fractions are spelled out and are used with hyphens (nine-tenths), mixed fractions are expressed in numerals, with the exception of cases when they stand at the beginning of a sentence. Decimals bring a few rules of their own: they are always expressed in figures, and sometimes are used without a zero in front of them (if the decimal proper starts with a zero, e.g., .01). Hyphens are also used in compound numerals from twenty-one to ninety-nine.
With large numbers there are no definite rules. Generally you should logically decide what way of writing them down is simpler. Round numbers are almost always spelled out, but again, make sure you are consistent within a sentence. If one number in a sentence is round and another is not, you should use numerals for both of them.
Position in the sentence is important as well. If the number starts the sentence, you should always spell it out, unless it violates the rule of consistency.
We think that this simple set of rules will be of great help the next time you encounter the necessity of using numbers in your text. Correct usage of numerals may seem like a small thing, but the overall success of a paper is a result of all these little details, and you are better off being aware of them.