Writing up your first paper and suddenly unsure whether to type "five" or "5"? It feels like a tiny question, yet numbers cause more small inconsistencies than almost anything else in a manuscript. The good news is that the core rule is genuinely simple, so let's start there.
Here is the short version first. In APA style, you spell out the numbers zero to nine, and you use numerals for 10 and above. That single rule covers most of what you will ever write. The exceptions are where authors slip up, and reviewers notice every one, so this guide walks through the main rule, the key exceptions, and the tricky details like decimals and large numbers.
But first, one reassuring point. These conventions come straight from the official APA Style guidance on numbers, so you are not learning our house rules; you are learning the standard the whole field uses.
The basic rule
Let's start with the core convention. Spell out whole numbers from zero to nine, and use numerals from 10 upward.
For example, you write "three participants" but "12 participants." The same rule applies to ordinal numbers, so you write "third" but "12th."
This is the APA and general scientific convention. Note, though, that other styles differ. Chicago style, for instance, spells out numbers all the way up to one hundred, so always confirm which style your journal or institution requires.
When to always use numerals
Some numbers take numerals no matter how small they are, even below 10. These exceptions matter most in research writing, because they come up constantly.
Use numerals in all of these cases:
- Units of measurement: 5 mg, 3 cm, 2 kg.
- Percentages: 4% of the sample.
- Statistics and maths: a ratio of 2:1, the 5th percentile.
- Time, dates, and ages: 3 years, 2 hours, a 6-year-old child.
- Scores and scale points: rated 4 on a 7-point scale.
- Exact sums of money: a $5 voucher.
- A specific place in a series: Chapter 4, Table 2, Figure 1.
Notice the pattern that ties these together. When a number attaches to a precise measurement or a label, it takes a numeral. So "2 participants per group" is correct even though 2 is below 10, because it works like a measurement.
When to always spell out
Other numbers take words, even when they are above 10. There are three main cases to remember.
First, spell out any number that begins a sentence, title, or heading. For example: "Seventeen studies met the criteria." Where that feels clumsy, simply reword the sentence so the number moves later.
Second, spell out common fractions, such as "two thirds of respondents" or "one half of the sample." Third, spell out numbers in universally accepted phrases, such as "the Twelve Apostles."
That first rule is the one authors forget most often. So say it to yourself once more: a sentence must never open with a numeral.
Numbers at the start of a sentence
This point deserves its own section, precisely because it trips up so many people. You cannot begin a sentence with a numeral, whatever the number is.
So you always have two options: spell the number out, or rewrite the sentence. For example, "35 patients withdrew" is wrong at the start of a sentence. Instead, write "Thirty-five patients withdrew," or reword it to "In total, 35 patients withdrew." For large numbers, rewording is usually the cleaner choice.
Decimals and the leading zero
Decimals follow one rule that confuses almost everyone at first. It concerns the little zero before the decimal point, and it is worth learning once and for all.
Use a leading zero when the number can be greater than 1. For example: "0.45 seconds," or "a weight of 0.73 kg." Do not use a leading zero when the number can never exceed 1. This applies to p values, correlations, and proportions, so you write "p = .03" and "r = .42," with no zero in front.
APA also recommends reporting most statistics to two decimal places, a convention the NIH Style Guide on numerals sets out clearly too. Our guide on presenting tables and figures covers how these numbers should appear in your data displays.
Large numbers and commas
Large numbers need consistent formatting as well, because a misplaced comma is a small but very visible error.
From 1,000 upward, use a comma to separate every three digits, as in 1,250 and 36,127. For very large round numbers, combine numerals and words for readability, so you write "3 million participants" rather than "3,000,000 participants."
One exception applies here. Do not use commas in page numbers, or in certain technical values such as some temperatures, so check your style guide for those specific cases.
Keep your numbers consistent
Consistency matters just as much as the individual rules. Reviewers quickly lose confidence when the same value appears two different ways in one paper.
So check that a number reads the same across your abstract, text, tables, and figures. Also apply the same number of decimal places within each measure. This is exactly the kind of detail that slips during revisions, so run a final numbers check after your last edit. Our guide on checking references and citations for accuracy describes a similar final-pass habit that catches these errors.
Common number mistakes
Scan this list before you submit, because each one is common and easy to miss.
- Starting a sentence with a numeral, such as "15 patients...".
- Spelling out a measurement, such as "five mg" instead of "5 mg".
- Adding a leading zero to a p value, such as "p = 0.03".
- Using numerals for numbers under 10 in ordinary prose, such as "5 studies".
- Inconsistent decimal places across a table.
- Forgetting commas in numbers over 1,000.
- Switching between words and numerals for the same value.
Individually, these are minor. Together, though, they quietly make a manuscript look rushed, which is the last impression you want to give an editor.
Your numbers checklist
Check these seven points before you submit:
- Numbers zero to nine are spelled out in ordinary prose.
- Numbers 10 and above use numerals.
- Measurements, percentages, and statistics always use numerals.
- No sentence begins with a numeral.
- Leading zeros appear only where a value can exceed 1.
- Commas separate every three digits from 1,000 up.
- Each value is formatted consistently throughout the paper.
Frequently asked questions
When should I spell out numbers in academic writing? In APA and most scientific styles, spell out whole numbers from zero to nine in ordinary prose, and use numerals for 10 and above. Always spell out a number that begins a sentence, a common fraction, and numbers in universally accepted phrases.
Do I use words or numerals in APA style? Use words for zero to nine and numerals for 10 upward. However, always use numerals for units of measurement, percentages, statistics, time, dates, ages, scores, and exact sums of money, even when the number is below 10.
Can I start a sentence with a number? Not with a numeral. Either spell the number out, such as "Thirty-five participants," or reword the sentence so the number appears later, such as "In total, 35 participants." Rewording is usually cleaner for large numbers.
When do I use a leading zero before a decimal? Use a leading zero when the value can be greater than 1, such as 0.45 seconds. Do not use one when the value can never exceed 1, such as a p value or a correlation, so you write "p = .03" and "r = .42."
How do I write large numbers in academic writing? Use a comma to separate every three digits from 1,000 upward, such as 36,127. For very large round numbers, combine numerals and words for readability, such as "3 million." Avoid commas in page numbers.
Do all citation styles treat numbers the same way? No. APA spells out zero to nine and uses numerals for 10 and above, while Chicago spells out numbers up to one hundred. The specific rule depends on your style guide, but consistency within your document matters in every style.
Conclusion
Numbers follow a clear system once you learn the exceptions. Spell out zero to nine, use numerals for 10 and above, and always use numerals for measurements and statistics.
Above all, stay consistent and never start a sentence with a numeral. These small choices quietly tell reviewers that you handled every detail with care, even on your very first paper.
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