Authors and professional writers can count on these tips to figure out the written rules of using a large number or a numeral.
SPELLING NUMBERS AS WORDS: These key guidelines for writing numbers, and rules for numerals, will help you learn how and when to express whole numbers and when to spell them in your business writing.
Few writers understand when you should use figures (aka digits) and when it is proper to write out the number in words - using letters.
For example, should you write the positive integer '5' as a figure, or do formal writing ethics dictate that it would be better to write the number 'five' spelled out as a word?
There are some basic rules to follow when you express arithmetical values in business letters, essays, and reports, to ensure you are using numbers and numerals in the correct way.
The key importance for writers is deciding whether to insert a full phrase, written as one hundred thousand three hundred and nine (for example), or should you use numerals and write it using digits such as 100,309?
The guiding rules for general writing suggest that you should almost always use full words for small numbers from one to nine (not 1 to 9) and numerals for those over nine. As a rule, use written words if it can be expressed in two words or less but remember that many will require a hyphen, such as thirty-nine.
However, the regulations vary and get a little confusing for even larger symbols. One rule that seems to be universally agreed upon is that you should never start a sentence with a number. To avoid torturing your readers, we also recommend rewriting a sentence instead of working with long digitized integers or a combination of complex numerals.
The key difference between a number and a numeral is a number is an abstract concept, whereas a numeral is a symbol used to express it.
For example the word 'three', the positive integer '3', and the numeral 'III' are all symbols which are used to express the concept of 'three-ness'.
In written English, we use the comma to separate thousands and as the period separator in decimals. The purpose of commas is to make large numbers easier to read.
For example instead of writing the size of the United Kingdom as 243610 square kilometers, it should be expressed as 243,610 square kilometers (or km²).
The International Systems of Units (SI) recommends a space to separate groups of three digits. The comma and the period should be used only to denote decimals, such as $14 300,75.
When you are writing about periods of time such as centuries or decades, they should be spelled out - such as during the 'Seventies' or in the 'twentieth century'.
You are allowed to use some digits with informal everyday writing. Therefore jotting a recipe as 'mix 3 cups of jasmine rice' is fine. Nevertheless, you should spell the percentage out like 'eight percent of the population' in formal communication. Expressing a percentage such as '8% of the population' should only be written for visual presentation such as in business use.
Tip: Spell out estimations! The simplest rule applies to numbers which are rounded up or estimated - use spellings. Those which are rounded higher than one million should be written as a numeral plus a word. Therefore, write 'around 300 million people' instead of 'around 300,000,000 people'. Exact numbers should be written out.
Two consecutive numbers placed next to each other is extremely confusing. Instead of scribbling something like '9 15-year-olds', write one as a numeral so it would become 'nine 15-year-olds'. It is easier to understand, and makes more sense, if you choose to script the number which has the fewest letters.
As a rule, it would be inconsistent to state 'She was my 1st true love'. Writing 'She was my first true love' is using extra consistency in the sentence especially when the cardinal numbers are related to each other. Spell it out if it is used in a quote such as '…four score..' and not '…4 score..'
Footnote: Expert authors and formal writers do not always agree on the rules for writing numbers and numeral. There does not seem to be one single standard that all literary professionals follow but we hope this section helped your understanding of the basic principles of spelling out numbers.
Numbers and Numerals; UK Rules Updated 2017