It is generally best to write out numbers from zero to one hundred in nontechnical writing. In scientific and technical writing, the prevailing style is to write out numbers under ten. While there are exceptions to these rules, your predominant concern should be expressing numbers consistently.
Numbers can disrupt readability in a paragraph, so for most writing purposes, it is best to flex those fingers and type out numbers less than 101 as fully spelled words.
When writing out numbers between forty and forty-nine, be sure to remember that forty has no u in it (this is a common spelling error).
The rules demonstrated in the examples above are simply rules of thumb and there are exceptions to them. For example, round numbers such as hundreds, thousands, or hundred thousands should be written out in full. Numbers that are not conveniently round will read better written as numerals.
When a number begins a sentence, that number should always be spelled out. That said, writers often choose to restructure their sentences when the numbers become cumbersome for the reader.
Spelling out numbers in technical, scientific, and complex writing
Scientific and technical journals, and even news reports, often adhere to the rule that only numbers less than ten should be written out in full, except when fractions or decimals are involved. This can be a sensible approach to ensuring the readability of texts that refer to numbers and figures frequently.
Fractional quantities of larger numbers, such as those in the millions and billions, are most easily read when abbreviated as decimals in combination with the word “million” or “billion” where possible. Whole millions and billions (and in American usage, trillions) can be expressed as a whole number plus the word “million,” “billion,” etc.
When using abbreviations for units of measurement in your writing, always express numbers as numerals. Similarly, when writing about money, use numerals in connection with the dollar sign or other currency symbols. Infrequent references to money read best when written out as spelled-out numbers plus the word for the currency involved.
Deciding whether to write out numbers in full can be tricky, but the key is to use the correct style for your audience and to use it consistently.