Most researchers already half-know the “spell out numbers below 10, use numerals for 10 and above” rule. What trips authors up in copyediting and peer review isn’t the base rule — it’s the exceptions layered on top of it, and the fact that the three style guides most commonly specified by journals (APA, the AMA Manual of Style, and the Chicago Manual of Style) don’t apply those exceptions the same way. This guide gives a single decision table for the situations that generate the most copyediting queries — units of measurement, percentages, decimals, sentence-initial numbers, ordinals, ranges, and large round numbers — plus worked before/after examples, so you can apply the correct convention without re-deriving it from the manual every time.
The short answer
- APA (7th edition) and the AMA Manual of Style (11th edition) both use a version of the same base rule: words for zero through nine, numerals for 10 and above — but numerals override that base rule whenever the number precedes a unit of measurement, reports a percentage, decimal, ratio, exact time/date/age, or statistical result, regardless of how small the number is.
- The Chicago Manual of Style (18th edition) is the outlier: its general/humanities convention spells out whole numbers zero through one hundred and uses numerals only above that. Chicago itself acknowledges a separate, more numeral-forward convention for scientific and technical writing that mirrors the APA/AMA pattern — which one applies depends on whether your journal or publisher has adopted Chicago’s general style or its scientific-writing guidance.
- Across all three, a number should never open a sentence, title, or heading as a numeral — spell it out or rewrite the sentence.
- A journal’s own Instructions for Authors always wins if it specifies a house rule that differs from the base style guide — treat everything below as the default to fall back on when a journal is silent.
Decision table: numeral or word, by rule
| Situation | APA (7th ed.) | AMA Manual of Style (11th ed.) | Chicago Manual of Style (18th ed.) |
|---|---|---|---|
| General number, zero–nine | Words (three groups, eight participants) | Words in general prose, unless another rule in this table overrides it | Words (Chicago’s general convention spells out zero through one hundred) |
| General number, 10 and above | Numerals (12 groups, 240 participants) | Numerals | Still words up to one hundred under Chicago’s general convention; numerals above 100. Chicago’s own scientific-writing guidance instead recommends switching to the numerals-at-10 convention throughout a quantitative manuscript — check which convention your publisher follows. |
| Sentence-, title-, or heading-initial number | Spell out, or restructure the sentence to move the number out of first position | Spell out, or restructure the sentence | Spell out (rarely an issue under Chicago’s general convention, since most numbers under 100 are already words, but still applies to numerals of 100+) |
| Number directly before a unit of measurement | Numeral, always (5-mg dose, 3 cm, 2 h) — even below 10 | Numeral, always (3 mL, 0.5 mg/kg) — the same override as APA | Numeral before an abbreviated unit, even under Chicago’s general convention (a 5 kg sample), though a spelled-out unit in running prose may still take a spelled-out number (five kilograms) |
| Percentages | Numeral + % symbol in running text (50%, a 5% increase) | Numeral + % symbol | Chicago’s general convention spells out “percent” in non-technical prose (50 percent); numeral + % is standard in tables, data-heavy prose, and Chicago’s scientific-writing guidance |
| Decimals | Always numerals, never spelled out (1.5, 2.27). Drop the leading zero for statistics that cannot exceed 1 — correlations, proportions, most p values (r = .34, p = .03) — but keep it for values that can exceed 1 (0.5 kg). | Always numerals. Confirm the leading-zero convention against the journal’s own reporting-statistics guidance, since practice varies by journal even within AMA-style publications. | Always numerals; leading-zero practice again varies by publisher — confirm against house style rather than assuming. |
| Ordinal numbers in narrative text | Words below 10 (the third trial), numerals from 10th up (the 11th participant) — the same threshold as cardinal numbers | Same pattern: words below 10, numerals from 10th up | Spelled out consistently with Chicago’s general cardinal-number convention (the twenty-first century) |
| Ranges | Numerals on both sides, with an en dash or “to” (5–10 participants, ages 18 to 65) | Numerals on both sides | Numerals if either number is 100 or greater or if the range reports an exact measurement; otherwise follows the general spell-out convention |
| Large, approximate round numbers | May combine words and numerals for an approximation (about 3 million responses) but use exact numerals when the figure is precise (3,241,092 responses) | Same distinction: word/numeral combination for an approximation, numerals for an exact count | Spelled out for a round, approximate figure (nearly a million); numerals for an exact or highly specific figure |
Where this table says “confirm against house style,” that’s not a hedge to avoid committing to an answer — it reflects real, documented variation between journals using the same base style guide. As the companion guide on research abbreviations notes for first-use conventions, style manuals set the default; a specific journal’s Instructions for Authors is the actual governing document whenever the two disagree.
Worked examples: before and after
Each pair below shows a common draft-stage phrasing and the corrected version under the applicable rule.
- Sentence-initial number. Draft: “12 participants withdrew before the follow-up visit.” Corrected: “Twelve participants withdrew before the follow-up visit.” (Or restructure: “Before the follow-up visit, 12 participants withdrew.”)
- Unit of measurement below 10. Draft: “Each sample received a five mg dose.” Corrected: “Each sample received a 5-mg dose.” — the unit overrides the below-10 spell-out rule under APA and AMA alike.
- Percentage below 10. Draft: “Response rates improved by eight percent.” Corrected (APA/AMA, or Chicago in a data-heavy context): “Response rates improved by 8%.”
- Probability/correlation decimal (APA). Draft: “The correlation was significant, 0.34, p = 0.03.” Corrected: “The correlation was significant, r = .34, p = .03″ — both statistics are bounded below 1, so APA drops the leading zero.
- Ordinal in narrative. Draft: “This is the 3rd time the protocol has been revised.” Corrected (APA/AMA): “This is the third time the protocol has been revised.”
- Mixed adjacent numbers. Draft: “We used 8 12-well plates.” Corrected: “We used eight 12-well plates.” — when two numbers appear back to back and would otherwise both be numerals or both be words, APA recommends spelling out the first to avoid visual confusion, keeping the unit-linked number as a numeral.
Frequently asked questions
Do tables and figures follow the same numeral rule as running text?
No — tables, figures, and statistical reporting conventionally use numerals throughout, regardless of value, since they’re read as data rather than prose. This mirrors the same “each stand-alone section resets the rule” logic documented for abbreviations: a table or figure is read in isolation, so it follows the numeral-dense convention appropriate to data display rather than the narrative spell-out rule.
What if my journal doesn’t specify a style guide at all?
Default to the convention dominant in your discipline — AMA for biomedical and clinical journals, APA for psychology and much of the social sciences, Chicago (or a discipline-specific variant of it) for humanities-adjacent social science and many university-press-published work. Whichever you choose, apply it consistently for every number in the manuscript rather than switching rule sets mid-document.
Should units always be abbreviated when paired with a numeral?
Under both APA and AMA, yes in most cases — see the full unit-abbreviation table in the SI units in scientific writing guide for which units are exempt from being spelled out and how to format them correctly.
Does the “numeral before a unit” override apply to approximate as well as exact quantities?
Yes — the override is about the presence of a unit, not about precision. “A 3 mg dose” and “roughly a 3 mg dose” both take the numeral; only the surrounding words change.
Where does this rule sit relative to other style conventions in a manuscript?
Number formatting is one of several sentence- and word-level conventions — alongside verb tense and abbreviation rules — that a copyeditor or reviewer checks for internal consistency before checking anything about the manuscript’s argument. Getting it consistent throughout, even more than getting every individual rule “right” on the first pass, is what actually reduces revision requests.
A consistency check before submission
Because number formatting rules interact (a number can simultaneously be “below 10,” “before a unit,” and “part of a range”), the most reliable way to catch inconsistencies isn’t memorizing every row of the table above — it’s a single pass through the manuscript specifically looking for numbers, checked against whichever style guide the target journal specifies. Do this pass after the manuscript is otherwise stable; renumbering or rewording during later revisions is a common source of new inconsistencies introduced after the original numbers check.
