How to write numbers, part 2: when to use hyphens

Things can get complicated when big numbers or long division are involved – not least when you have to write numbers out. The question even extends to the punctuation you should use – especially hyphens. For instance, do you need hyphens in long numbers when you write them as words? Or in ages? What about in a fraction, like ‘two-thirds’?

These might sound like arcane questions, but for anyone who has to write about numbers it could be one that comes up all the time. We get asked about it regularly too. And, after all, helping to shed light on the more obscure points of punctuation is all part of our job.

The answers may seem complicated, but don’t worry: much like your times tables, a lot of this is just about rules you have to learn. And you won’t even have to stand up in front of the class to recite them.

Hyphens in fractions

Use hyphens in fractions whenever they are written as words, whether they function as a noun (two-thirds is more than one-half), adjective (a two-thirds majority) or adverb (two-thirds finished).

In fact, opinion is divided over whether it is necessary to hyphenate fractions when they are nouns, so it does come down to your organisation’s house style.

However, while the possibility of ambiguity from leaving out the hyphens may be small, there are some areas for minor comical confusion, particularly in US English:

He had a wallet stuffed with dollars and small change; now he’s spent three quarters.
They had a full case of whisky, but they’ve drunk two fifths.

Hyphens in adjectives with numbers

Hyphens join words that work together as a unit to describe something that follows them in the sentence, eg an easy-to-read report. This is called a compound adjective (or compound modifier). Where the description follows the noun, you won’t need the hyphens: the report was easy to read. The same rules apply when numbers are part of the description, and whether the numbers are cardinal (one, two, three …) or ordinal (first, second, third …).

It’s a three-and-a-half-page report.
The report is three-and-a-half pages.
There are too many pages in the report – I stopped reading after two and a half.
They are our second-biggest client.
This client is our second biggest.

Depending on your company’s style, you might use figures rather than words for some or all numbers and fractions. But you’d still use hyphens in the same way in descriptions.

a 60-second fix
a 3 ½-page report

You can see that where there is a mixture of whole numbers and fractions in figures, you treat them as one unit and don’t put a hyphen between them.

Hyphens in ages

Hyphens in ages work the same way as in the any other compound adjective (see above), where words work together to describe something: if the age is before the noun, use hyphens. If it’s after, don’t.

She’s a two-year-old girl [all three words work together to describe the girl].
The girl is two years old.

Where you refer simply to a two-year-old, use hyphens throughout. The noun it refers to – girl, boy, cat, or whatever – is absent, but implied. However, you’d need to make it clear by the context what that noun actually is.

Why is this important? Well, consider that there’s quite a big difference between twenty-four-year-olds and twenty four-year-olds.

Hyphens in percentages

You don’t need to use hyphens in percentages unless they form part of a longer description (compound adjective) before the noun. Again, this is just another version of the rule about compound adjectives above.

a 12 per cent increase in sales
a 12-per-cent-a-month increase
an increase of 12 per cent a month

Suspended hyphens to show range

If you need to show a range, you can use suspended hyphens (also known as hanging hyphens) for the description. For example, if you have a two-month-long project which you now expect to take between two and four months to complete, you can avoid repeating ‘month-long’ by leaving your first hyphen hanging, like so:

a two- to four-month-long project

Hyphens in numbers as words

When to use words for numbers and when to use figures is a style choice. Quite a few style guides (including ours) suggest using words for the numbers one to ten, and figures for 11 and up. But these guidelines do vary across organisations.

So check if there’s a style in place where you work. And if you do need to write out more numbers as words, you can follow these rules.

Use a hyphen when writing two-word numbers from twenty-one to ninety-nine (inclusive) as words.

But don’t use a hyphen for hundreds, thousands, millions and billions.

For example:

thirty-two (32)
two hundred and thirty-two (232) [note that in US English you might omit the ‘and’ between the hundreds and the tens and units]
two thousand two hundred and thirty-two or twenty-two hundred and thirty-two (2,232)

To sum up

As you’ve probably noticed, some of these rules are of the ‘you just need to know them’ type, and others come down to a style choice. Either way, you’re not actually back at school, so you can always refer back to this page as often as you need to. (Or write the answers on your hand, if you like.)

And if you’ve any questions left over, just leave them below.

For the basics of writing numbers, figures, dates and times, see this post.

Image credit: 2013-12-28: (361/365) Chicken Foot Dominoes P1170131 by Dave Lundy used under CC BY-ND 2.0 / cropped from original

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Author: Catie Holdridge

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Catie joined Emphasis in 2008 with an English literature and creative writing degree under her belt. She is production editor and co-writer of our online training programme, Emphasis 360; she's also written numerous articles for the Emphasis blog. And she keeps office repartee at a suitably literary level.