You type "the team hired three new editors" in one paragraph. Two paragraphs later you type "we shipped 14 features this quarter." Both sentences look fine on their own, and that is exactly the problem: the document has quietly switched notation without telling anyone, and a sharp editor will notice before your reader ever does.
Numbers are the small print of style. Everyone assumes they know the rules, almost nobody can state them aloud, and the rules turn out to be habits that vary from publisher to publisher. Here is how those habits work, so you can pick one and stop second-guessing every digit.
The uncomfortable truth: there is no single threshold
Ask ten editors when to spell a number out and you will get confident, contradictory answers. One will say spell everything under ten. Another will say under a hundred. A third will say it depends on what the number is doing in the sentence.
All of them are quoting a real convention. That is the catch. There is no universal line between words and numerals, and anyone who tells you otherwise has internalized one publisher's house style and mistaken it for the language.
So the threshold matters less than you think. A document that writes "nine" in one sentence and "9" in the next is sloppy, even though both choices are defensible on their own. Consistency is the rule; the threshold is just the setting you picked for it.
The thresholds the major guides use
Style guides fall into three camps. The tightest spells out numbers up to nine and switches to numerals at 10, common in journalism and web writing where numerals scan easily. A middle camp spells out up to ten, which you will find in many house styles and corporate documents. The loosest, closest to Chicago-style practice, spells out up to one hundred, which gives you "the committee approved ninety-four applications": fine in a literary essay, strange in a spreadsheet summary.
| Number | Tight style (up to nine) | Loose style (up to one hundred) |
|---|---|---|
| seven | seven | seven |
| twelve | 12 | twelve |
| ninety-four | 94 | ninety-four |
| 1,200 | 1,200 | 1,200 |
| 3.5 million | 3.5 million | 3.5 million |
Notice that by the time you reach four digits, everyone agrees. No serious guide wants "one thousand two hundred" in running prose. The disagreement lives almost entirely in the low double digits, which is where most writers waste their anxiety.
The one rule almost everyone shares
Here is a convention with near-total agreement: never begin a sentence with a numeral.
"The 12 participants withdrew" is wrong under almost every style guide. The reason is practical. A sentence is a unit of prose, and "12" at the start looks less like a number than a stray label at the top of a paragraph.
You have two fixes:
- Reword so the number is not first: "Twelve participants withdrew."
- Add a short lead-in: "In total, 12 participants withdrew."
The second option is the better long-term habit, because it rescues you from the genuinely awful case: a sentence that must open with "1,340,000." No reader wants to parse that at the top of a paragraph.
Large numbers, millions, and money
Once numbers get big, the question shifts from words versus digits to how much precision the reader actually needs.
Round large figures and use the shorthand people already think in: "3.5 million," "roughly 80,000," "just under half a billion." Writing "3,500,000" mid-paragraph forces the reader to count zeros, which is work prose should never demand.
For money, the same principle applies. Use the symbol with the numeral ($45, not "forty-five dollars") for any figure your reader will scan, and reserve spelled-out amounts for formal contexts like contracts and checks, where the words exist to prevent tampering.
Two traps deserve flagging. Keep "million" singular ("3.6 million," never "3.6 millions"). And decide once whether you write "USD" or "$", then hold to it, because mixing them signals that two different people wrote the piece.
Ranges, dates, decades, centuries
Ranges have their own small doctrine, and it mostly comes down to how much you repeat.
| Case | Conventional treatment | Why |
|---|---|---|
| Consistent unit | 20-30 pages | Repeat the unit once, at the end |
| Percentages | 15 to 20 percent | Repeat the word, keep both numerals |
| Money | $5-$10 million | Repeat the symbol, keep values short |
| Years | 2019-2023 | No apostrophe, full four digits |
| Decades | the 1990s, the '90s | Apostrophe replaces the missing digits |
| Centuries | the twenty-first century | Spelled out and lowercase in prose |
Decades take an apostrophe only where digits are missing: "the 1990s" needs none, "the '90s" needs one. And it points in one direction only. "The 1990's" is not a contraction of anything; it is a typo with staying power.
Centuries are spelled out and lowercase in formal prose (the twentieth century), while numerals work in tables and headings. If you want the full rules for hyphens inside constructions like "twenty-first," the compound modifier guide covers the fiddly cases.
Ordinals, units, and the technical exception
Ordinals follow whichever threshold you chose for cardinals. Under a tight style: 10th. Under a loose one: tenth. What you should not do is invent a hybrid, writing "first" in one sentence and "4th" in the next.
For units and measurements, technical writing breaks from all of this. There, numerals appear at every size, because the numbers are data rather than prose: "a 5 mm gap," "2 mL of solution," "37 °C." Nobody wants to read "five milliliters of solution" in a lab report.
The dividing line is purpose, not pomposity. If a number carries a measurement someone will act on, use numerals and symbols. If it is part of a sentence a person reads for meaning, spell out the small ones. The readability guide is a useful companion when you are writing the same material for two very different audiences.
Frequently Asked Questions
Is it "nine" or "9" in formal writing?
In formal prose, most guides prefer the word for small numbers, up to nine or ten, and numerals above. The more useful answer is that your document's own consistency matters more than either choice. If you spell out "nine," spell out "seven."
Should I write "percent" or the % symbol?
In running prose, most guides prefer "percent" spelled out. In tables, charts, and scientific writing, the symbol is standard and saves space. Pick one per document and never mix them inside the same paragraph.
How do I write a range of numbers?
State the unit once and let it serve both ends: "20-30 pages," "15 to 20 percent." Repeating the unit after both numbers ("20 pages to 30 pages") is clunky, though some legal documents do it deliberately for precision.
Do I need a comma in "1,200"?
Yes, in English-language publishing, four-digit numbers and above take a comma as a thousands separator. Much of continental Europe does the opposite, using a comma as a decimal point, so check the audience before handing a document to an international team.
Pick a lane and write it down
Here is the only move that ends the argument. Decide your threshold once, today, and write it into a style sheet alongside your date and currency formats. Nine or ten, symbols or words, percent or %. Then apply it without exception, even where another guide would have chosen differently. A find and replace tool will check every numeral in about a minute, and a character counter confirms nothing else drifted. The reader never notices a rule you applied well. They notice the moment you break your own.