While my official designation is “technical writer,” I actually earn the bulk of my pay these days editing stuff other people have already written. As a result, I start to understand the frustrations some editors face when reviewing someone else’s work. Today I’d like to discuss the use of numbers in documents–when to spell them out as words and when Arabic numerals will suffice. Onward!

Where Are These Rules Are Coming From, Anyway?

I’ll be honest: I’m not going to quote grammar websites or textbooks. These guidelines are based on my experience with in-house styles at Disney, the Department of Defense, and NASA, where I had other editors reviewing my work. Most everywhere else, there haven’t been any printed rules, so I’ve just imposed the rules I internalized from the aforementioned organizations.

As for why we spell numbers vs. write them out as words, the best explanation I can offer is that it usually improves readability. Try these examples:

• I saw 1 cow, 2 horses, and 3 pigs.
• The solid rocket booster for the Space Launch System generates one million six hundred thousand pounds of thrust.
• There are one hundred and forty-four eggs in a gross.
• The customer said she wanted to buy 2 computers.
• The universe is estimated to be thirteen billion, seven hundred and eighty-seven billion years old, plus or minus twenty million years in either direction.

Okay, let me state here that none of the bulleted sentences above are wrong, per se. They’re just hard on the eyes because they cause the reader to slow down…this reader, anyhow. In all of the cases above, it has become standard practice to use numerals (or other notation) for long numbers and words for smaller numbers.

To make life easier, someone came up with the brilliant notion of setting a limit:

• 1-10: Spell out
• 11+: Use the numerals

But What If I’ve Got Large and Small Numbers in a Sentence?

Use numerals throughout that sentence. Once out of that sentence, though, it’s time to move back to spelling out one through ten.

What About Ordinal Numerals?

Ordinal numerals are what you use when you are putting things in order: first, second, third, fourth, fifth, etc.

Again, for numbers up to ten, especially if you are using them in body text (a standard sentence or paragraph as opposed to a table, graphic, or heading of some sort), you still spell out the words in English vs. numeric form. If you’re going to have a long list of things to discuss–beyond ten items, anyway–you’re better off numbering the paragraphs from the start rather than including words in your laundry list (“15. I demand dark chocolate candy bars with every meal.”).

What If the Name of an Item Includes a Number?

By all means, if you’ve got a number in a proper noun (or even some common nouns–I’ll get to those in a moment), use the number:

• F-100 Super Sabre
• CST-100 Starliner
• Ginsu 2000 Kitchen Knives

You can also run into numerals in lower-case situations. For example, I edit a lot of multi-day training courses. In situations like that, it makes sense to use “Day 1” or “Day One,” depending on which standard you set up front–just be consistent.

Another good place to use numbers is when dealing with currency or units of measure:

• $5.89
• €10.00
• 2 kilometers
• 50 tonnes

If you’re writing a story, it might make sense in dialogue to say, “The guy owed me five dollars.” If you’re dealing with much larger numbers–which happens in science fiction occasionally–you might pick a better way to make your point. Is the number crucial to the point you’re making? If not, find another way to express it.

What About Roman Numerals?

The only time we use Roman (I, V, X, D, C, M) instead of Arabic numbers (1, 5, 10, 100, 500, 1,000) is usually for numbering sections of a large document. We don’t spell them out, and if we had cause to, it’s best to treat them as Arabic numerals; meaning you wouldn’t spell out eye for I, vee for V, ex for X, etc. You’d still spell them out as one, five, ten, etc. Most of the time, documents with more than ten sections or chapters won’t use Roman numerals anyway, unless they’re using an older style anyhow. The last novel I recall reading with Roman-numbered chapters was David Copperfield.

Likewise, if you’re using ordinal numerals to describe a Roman-numbered list, you’d still say, “First, second, third, etc.” not primarius, secunduri, tertiarum, etc.

Also, if you’ve included a Roman numeral in a proper noun, by all means, use the letter/number:

• Delta IV
• Constitution II
• Louis XVI

Fortunately, the ability (and need) to read Roman numerals is declining–I only learned them to figure out when movies were made, or in the case of the last individual on the list above, to find out the birth order of a particular monarch.

Does It Have to Be This Complicated?

Alas, for now, yes. Just keep it simple in your head: spell out one through ten, use numbers for anything else. Make that grammar count! 🙂