When learning a new language, students almost always begin with the alphabet and numbers. We use letters, of course, to form words, which form sentences that express ideas of varying complexity in a form that people who read this written language can understand. Numbers designate a different kind of language, one that conveys equations and measurements, mathematical calculations and scientific formulas. But sometimes the two forms come together, when, for example, we are outlining how to follow numerical steps or stating someone’s age or simply noting that there is more than one right way to write numbers. How and when are we supposed to put numbers into words?
Generally, the rule of thumb for writing numbers in English prose goes like this: spell out the word for numbers under ten, and write the numerical symbol for numbers 11 and higher.
For the party, I bought two frozen pizzas, three quarts of ice cream, and six bags of chips. I also bought a 12-pack of seltzer.
The Gregorian calendar comprises 12 months or 356 days, and each month has 28, 30, or 31 days.
As with most rules in English, however, there are exceptions.
The first word of a sentence must always be spelled out, even in the case of large numbers. For instance:
Forty students are trying out for the volleyball team this fall. (correct)
NOT: 40 students are trying out for the volleyball team. (incorrect)
For the sake of consistency and style, it is often advisable to keep the same format for writing out multiple numbers within a sentence, even when combining large and small numbers:
On our cross-country road trip, we visited 22 cities in 9 different states.
(The inconsistent alternative would be 22 cities in nine states.)
Similarly, if numbers appear many times throughout a piece of text, it is better to write them all numerically for ease of reading.
There are also specific rules based on different contexts for writing numbers.
Ordinal numbers, as the name suggests, are used to express numerical items in relation to each other, or in a certain order, according to their quality or placement. In English, ordinal numbers are spelled differently from their cardinal number counterparts. (A cardinal number is what we typically think of when we think of counting numbers – in other words, numbers that denote a quantity: one, two, three, etc).
You can recognize ordinal numbers by their suffixes, which apply both in the spelled out word and the numerical abbreviation of ordinal numbers.
One → first (1st)
Two → second (2nd)
Three → third (3rd)
Four → fourth (4th)
Five → fifth (5th)
Six → sixth (6th)
Seven → seventh (7th)
Eight → eighth (8th)
Nine → ninth (9th)
Ten → tenth (10th)
Eleven → eleventh (11th)
Twenty → twentieth (20th)
Deciding whether to spell out ordinal numbers or write the numerical abbreviation usually depends on the length of the number. When it is only one word (like second), spell it out: I won second place at the spelling bee!
When the cardinal number is composed of multiple hyphenated words (like thirty-one), the ordinal number can be expressed as a numeral: The office is located on the 31st floor.
Writing dates is another context in which we inherently combine words with numbers (to describe the name of a month and the number of a day or year).
When writing out the full date of an event, use the cardinal number:
The celebration will take place on August 21, 2019.
When referring to a specific day without mentioning the year, use the ordinal number:
We are all getting together on the 21st of August to celebrate!
When referring to a person by their age, the most important thing to know is where (and when) to add hyphens.
When the age functions as a noun (referring to a person by their age) or as an adjective (describing a person or object), you need to hyphenate:
The 68-year-old was known for being an incredible chef. (noun)
The 68-year-old chef retired after working in the same restaurant for 30 years. (adjective)
Whether or not to spell out a person’s age depends on the size of the number, following the first rule of thumb. For ages under ten, spell it out: The girl is seven years old, and she has a four-year-old brother.
For ages higher than ten, you can write the numeral: My grandfather was 68 years old when he retired.
How and when to follow these rules about writing numbers is ultimately a stylistic choice, and different publications may have their own set of expectations.