Proofreading Tips: When Should You Write Out Numbers in Full?

Style guides vary a lot on even the basics of how to write numbers.

APA style, for instance, recommends writing out numbers under 10 as words and using numerals for larger numbers. But Chicago style recommends writing out numbers up to and including 100. And MLA style recommends writing out any number that can be written as one or two words. All of these styles, moreover, include plenty of exceptions based on how and where numbers are used (e.g., APA style always uses numerals for time expressions, even for numbers under ten).

This variation is why it is vital to check your client’s style guide as a proofreader! But what do you do when your client doesn’t have a style guide? Or if their style guide doesn’t provide guidance on how to present numbers in the text? To help you decide when numbers should be written out as words and when they should be written with numerals, we’ve compiled a few helpful guidelines below.

Writing Numbers: Words or Numerals?

If your client does not have a style guide, the most important thing, as ever, is to ensure a clear, consistent style is used. But there are also some helpful rules of thumb that you might want to draw on. For instance, we recommend the following as a starting point:

• Write out whole ordinal and cardinal numbers under 10 as words.
• Write whole ordinal and cardinal numbers 10 and above as numerals.

We can also pick out some broad guidelines for specific contexts where exceptions may apply. For example, most style guides would recommend using numerals in the following situations:

• Mathematical and statistical expressions.
• Decimal numbers (e.g., 2.5, 0.16) and mixed fractions (e.g., 2 ¾).
• Measurements in technical and scientific writing.
• Addresses and street names (e.g., 35 New Street, 81 5^th Street).
• Years (e.g., 1968, not nineteen sixty-eight).
• Proper names that include numerals (e.g., 20th Century Studios).

And most style guides suggest using words in the following contexts:

• When numbers occur at the start of a sentence.
• Inexact casual and idiomatic uses (e.g., I told him a hundred times).
• Proper names that present numbers as words (e.g., Fifth Avenue).

These guidelines can provide a basis for ensuring numbers are used clearly and consistently in your client’s work. However, you may also need to consider variations on these rules, since whether numbers are best written as words or numerals may depend on the context. In the rest of this post, we will look at some key considerations in this respect.

Large Numbers

Large round numbers are often written as just words (e.g., two thousand, one million). This is because they are usually easier to identify as words than as numerals: e.g., 1,000,000 and 10,000,000 look similar written down at a glance, so readers could get such numbers confused, but the terms one million and ten million are more obviously distinct.

However, it is also acceptable to mix numerals and words in these cases, particularly when discussing millions, billions, and so on (e.g., 5 billion). And it may even be necessary to avoid awkward phrasing when a large number includes a decimal (e.g., 5.2 billion).

Remember, though, that these guidelines only apply to approximate or round numbers. Large exact numbers should always be written as numerals (e.g., 1,248,103).

Fractions and Percentages

Fractions and percentages are most often written with numerals, especially in mathematical contexts. And mixed fractions (e.g., 2 ¾) and complex fractions are almost always written as numerals.

However, simple fractions can also be written out as words. This is especially common when they are used casually or inexactly:

He ate almost ½ of the cake in one sitting. ✘

He ate almost half of the cake in one sitting. ✔

And while percentages should always be written as numerals when accompanied by the % symbol, there is room for variation when used alongside the word per cent or percent:

The process is currently five % complete. ✘

The process is currently 5% complete. ✔

The findings show that 48 percent of people agree. ✔

Our research examined eight percent of the responses. ✔

If your client doesn’t have a style guide, then, all you’ll need to do is check that fractions and percentages are expressed clearly and consistently.

Measurements and Currency

We mentioned above that measurements are usually expressed as numerals in scientific and technical writing. In addition, most style guides recommend using numerals whenever numbers are used alongside abbreviated units of measurement:

The experiment used eight mm mounting threads. ✘

The experiment used 8 mm mounting threads. ✔

In other contexts, though, there is room for variation. For example, if a unit of measurement is written out in full, it is typically fine to use either numerals or words to express the accompanying value:

We went on a twelve-mile walk yesterday. ✔

We went on a 12-mile walk yesterday. ✔

The fabric strip is five centimeters long. ✔

The fabric strip is 5 centimeters long. ✔

Whether to present such numbers as words or numerals therefore depends on what is clearest and most suitable in context (and what will be consistent with the rest of the document).

Similar guidelines can be applied to currency. In other words, amounts of money should always be written with numerals when used alongside a symbol. But it is generally fine to use either numerals or words when the currency is written out as words. For example:

It cost me $45. ✔

It cost me forty-five dollars. ✔

I only have five dollars right now. ✔

I only have 5 dollars right now. ✔

Again, the most appropriate form will depend on what is clearest in context and whether it is consistent with the rest of the document (e.g., if the author has typically written out numbers under 10 as words, then it would make more sense to say five dollars than 5 dollars).

Times and Dates

Numerals are commonly used in times and dates. For example:

The meeting begins at 11:15 a.m.

The final deadline is September 12, 2022.

In most cases, moreover, numerals will be the clearest way of expressing these things. But, as ever, there are situations in which using words may be equally acceptable. When writing times, for example, it is common to use words alongside the term “o’clock”:

We’ll meet at one o’clock on Thursday.

And dates can be written using just numerals, just words, or a mix of both:

The final deadline is September 12, 2022.

The final deadline is 9/12/2022.

The final deadline is September twelfth, 2022.

We typically recommend using numerals for the date and year but writing the month as a word for the optimal combination of brevity and clarity, especially if a purely numerical format may be unclear (e.g., 9/12/2022 reads as September 12 for a US audience, but it could be read as December 9 by people who are accustomed to the UK date format).

However, as ever, the key factors here when proofreading are clarity and consistency. And as long as the usage works in context, times and dates can be presented with either words or numerals.

Decades and Centuries

Like individual years, decades are usually presented in numerals when written out in full:

This was common in the nineteen seventies. ✘

This was common in the 1970s. ✔

However, decades can be presented as words or numerals when they are abbreviated to exclude the century:

This was common in the seventies. ✔

This was common in the ‘70s. ✔

And centuries can be written using either numerals or words:

The 20th century was a time of great change. ✔

The twentieth century was a time of great change. ✔

Other than when a decade is written out in full, then, there is room for flexibility here. And, as a proofreader, your main concern will be ensuring clarity and consistency in these cases.

Starting a Sentence with a Numeral

As mentioned near the start of this post, most style guides recommend avoiding numerals at the start of a sentence. But what should you do if you see a sentence that starts with a numeral?

One option is to write it out as a word instead. For instance:

52 kittens were in the basket. ✘

Fifty-two kittens were in the basket. ✔

Another option is to rephrase the sentence so it no longer starts with a number. This can be especially helpful when writing out the number as words would be awkward (e.g., large complex numbers and decimals):

4.85 people on average in each group scored full marks. ✘

On average, 4.85 people in each group scored full marks. ✔

There may also be occasions when starting a sentence with a numeral is the best option available (e.g., if rephrasing or writing the number out in full would both be awkward). Generally, though, it is better to correct or flag numerals at the start of sentences where possible.

Consistency within Passages and Sentences

Another common reason for bending the rules about writing numbers is to maintain consistency within a sentence or passage of text. For example:

The first test involved only five participants, but the second included 25.

Here, we have two numbers in a single sentence. The first (five) is written as a word. The second (25) is written as a numeral. This is not necessarily a problem, and some style guides recommend this approach. However, both numbers refer to the participants in a study. And since these are equivalent uses, many style guides would recommend using a consistent style for the numbers to make this clearer and enable comparison.

In such situations, it may therefore make sense to switch to whichever form will be clearest for both usages. And in most cases, this will mean using numerals consistently:

The first test involved only 5 participants, but the second included 25.

This isn’t to say that numbers in a sentence always have to be consistent. In fact, sometimes it helps to use numerals for one category of numbers and words for another. For instance:

Five participants were 15 years old or younger, while the other twenty were over 16.

Here, for example, we use numerals for the ages of participants, but we use words for the number of participants. This helps to ensure that the different types of numbers are distinct.

So, while consistency is usually helpful when multiple numbers appear in a single sentence or passage, clarity is also a factor. And whether you need to make a change to ensure consistency (or to vary how numbers are written) will depend on which approach is clearest.

Becoming a Proofreader

We hope that you’ve found the guidelines above useful, but keep in mind that they are only guidelines! There is a lot of room for variation here. And if your client is using a style guide, that should be your first point of reference for how numbers should be presented.

If you want more in-depth information about the English language and how proofreading works, though, why not try our Becoming A Proofreader course? It teaches you everything you need to know to begin or develop your proofreading career. And with a free trial available, you can see how Becoming A Proofreader can help you today.