A fundamental guideline for writing numbers is that little numbers from one to ten (or one to nine, depending on the style guide) should be spelt explicitly. Larger numbers (those more than 10) are written as numerals. There are two forms of the numeral 1: 'one' and 'unit'. A unit can be another number or a single symbol, such as a '$' or '%'.
In English, there is no difference in meaning between spelling out '1' and using the numeral '1'. However, in some styles of writing, it is preferred to use numerical symbols instead of words for large numbers. For example, '99 bottles of beer on the wall' would be written as '99B'.
It is acceptable to use digits other than those available in word form, but they must be interpreted by context. For example, if a recipe calls for '3 cups of flour', you would assume that it means any whole number of tablespoons, not just three. If there are measuring instruments available, the ingredients should be measured to the nearest approximation of the quantity being prepared.
Numbers can also be expressed as fractions. For example, 'a quarter of the cake' could be written as 'quarter of a cake'. But because four makes up half of six, this can be simplified to 'half of a cake'.
Making Numbers Numbers up to nine should always be expressed in words, whereas numbers more than nine can be written in numerals. For higher numbers, depending on the context, you can use either digits or words (e.g., a thousand people or 1,000 people), but in technical writing, you should always use figures, e.g., 200,000 km.
In nontechnical writing, it is often better to write numbers from 0 to 100. The dominant technique in scientific and technical writing is to write out numbers less than 10. While there are exceptions to these guidelines, your primary focus should be consistently expressing numbers.
Above that level the best approach depends on the context.
There are three main approaches: zero-based numbering, offsetting counters, and arbitrary numbering. Zero-based numbering starts with zero and adds one for each item counted. So, the first item gives a result of 1, the second item gives a result of 2, and so on. This is the most common method used in science and technology contexts. Offsetting counters start with a given value and then subtract from this number at each occurrence. So, the first occurrence increases the counter by 1; the second occurrence decreases the counter by 1, and so on. This method is commonly used in business contexts. Arbitrary numbering starts with any number and then increments or decrements it each time the term is used. So, the first occurrence will give a result of 1, the second occurrence will give a result of 2, and so on.
Which method is used depends on the context and who is using it. If you are using zero-based numbering to count objects, then it makes sense to use that in scientific writing too.
Because numbers might disrupt reading in a paragraph, it is better to flex those fingers and type out numbers less than 101 as completely spelt words. When a number starts a statement, it should always be spelt properly. Examples are 27 years old or 27th January.
The Evidence Is in the Numbers.
Fortunately, the rules for writing about numbers are far simpler—and more fascinating—than the meanings of terms connected to numbers. The Chicago Manual of Style, The Bluebook, and the ALWD Guide to Legal Citation all recommend spelling out numerals ranging from 0 to 99. Why? Because writers who use abbreviations risk causing confusion for their readers.
Abbreviations are easy to misunderstand because they often resemble the things they stand for. For example, a writer might refer to "a case cited by" or "a statute referenced by" a court when what she really means is "Case No. 123456789." Or she may simply mean to say "see Case No. 123456789." When that happens, her readers will have no way of knowing which case she's talking about!
Numbers greater than 99 are usually not spelled out unless they are part of a series (e.g., November 30, 1999). Even then, it's acceptable to use abbreviations as long as they're defined later in the text. For example, if I were referencing the second Thursday in October, I would write that date as 10/2/1999 instead of using the abbreviation "10/02/99".
Finally, if you must use an acronym, define it before using it for the first time.