Numbers, Numerals, and Units of Measure in Academic Writing: Rules and Examples

Key Takeaways

• Most style guides follow a version of the ten-rule: spell out numbers below 10 and use numerals for 10 and above, but APA, MLA, Chicago, IEEE, and Vancouver each apply exceptions differently.
• SI units always pair a numeral with a symbol, use a space before the symbol, drop the period, and never take a plural s, as in 5 kg rather than 5kgs.
• Numbers inside chemical compound names, such as locants and multiplying prefixes, follow IUPAC nomenclature rules rather than the number rules used in ordinary prose.
• Gene symbols are italicized and often include numbers that mark family members or variants, while the corresponding protein names are usually written in plain, non-italic text.

Contents

• General Principles for Writing Numbers in Academic Text
• How Do Style Guides Differ on Numeral Rules?
• Units of Measure and the SI System
• Numbers in Chemical Compound Names
• Numbers in Gene and Protein Names
• Do All Style Guides Handle Number Ranges the Same Way?
• Common Mistakes to Avoid in Academic Number Usage
• Frequently Asked Questions

General Principles for Writing Numbers in Academic Text

Academic writing relies on numbers constantly, whether reporting a sample size, a measurement, or a percentage. Because inconsistent number formatting distracts readers and can even create ambiguity in a research paper, every major style guide sets rules for when to spell out a number and when to use a numeral. These rules share a common foundation but diverge in the details, so writers need to know which guide applies to their discipline and journal.

When to Spell Out Numbers

• Numbers below 10 in most general and humanities styles, such as three participants or seven trials.
• Any number that begins a sentence, title, or heading, regardless of its size.
• Common fractions used in a general sense, such as one-third of respondents.
• Approximate or rounded figures expressed informally, such as about a hundred years ago.

When to Use Numerals

• Numbers 10 and above in most academic and scientific styles.
• Any number that is paired with a unit of measurement, such as 5 mL or 12 kg.
• Percentages, ratios, and statistical or mathematical values, such as 8% or a ratio of 3:1.
• Dates, times, ages, page numbers, figure numbers, and monetary amounts.
• Numbers presented together for comparison, even if some fall below 10.

How Do Style Guides Differ on Numeral Rules?

Each style guide sets its own threshold for spelling out numbers and lists its own exceptions. Most guides default to words below 10 and numerals from 10 upward, but APA, IEEE, and Vancouver push numerals lower for units, statistics, and measurements, while MLA and Chicago allow more flexibility for general prose.

Style	Below 10	10 and Above	Key Exception
APA	Spell out	Numeral	Numerals for units, statistics, ages, and time
MLA	Spell out one or two-word numbers	Numeral if more than two words	Numerals throughout technical or data-heavy text
Chicago	Spell out zero to one hundred	Numeral above one hundred	Scientific papers may switch to the ten-rule instead
IEEE	Numeral in most cases	Numeral	Spell out only at the start of a sentence
Vancouver	Spell out	Numeral	Numerals whenever a unit or measurement follows

APA Style

APA, now in its seventh edition, uses the ten-rule as its baseline: numbers below 10 are spelled out and numbers 10 and above are written as numerals. Several exceptions override this rule and push a number into numeral form even when it is below 10.

• Numbers that precede a unit of measurement, such as 5 mg or 3 km.
• Numbers that represent statistical or mathematical functions, sample sizes, or scores.
• Numbers that denote a specific time, date, or age, such as 3 years old.
• Numbers in a series where at least one number is 10 or above.

MLA Style

MLA style, widely used in literature and the humanities, spells out any number that can be written in one or two words, such as three or two hundred. Numbers that need more than two words are written as numerals, such as 132. MLA also permits an author to switch entirely to numerals in technical writing that contains many figures, as long as the choice stays consistent throughout the document.

Chicago Style

Chicago style offers two systems. Its traditional rule, common in the humanities, spells out whole numbers from zero through one hundred and uses numerals above that. Chicago’s guidance for scientific and technical writing instead recommends the ten-rule familiar from APA, so writers in the sciences using Chicago style should confirm which system their publisher or journal expects.

IEEE Style

IEEE style, used in engineering and computer science, favors numerals almost everywhere because these fields rely so heavily on quantitative reporting. A number is spelled out only when it opens a sentence, and writers are generally advised to reword the sentence so it does not begin with a numeral at all. Numerals are always used with units of measurement.

Vancouver Style

Vancouver style, common in biomedical and clinical journals, closely resembles the conventions used in AMA style. Numbers below 10 are spelled out in general prose, but numerals are used whenever a number precedes a unit, appears in a percentage, or reports a measurement, dose, or statistical result. This keeps clinical data unambiguous for readers scanning results quickly.

Units of Measure and the SI System

What Is the International System of Units?

The International System of Units, abbreviated SI, is the modern metric system used worldwide in science to standardize measurement units such as the meter, kilogram, and second. Nearly every academic style guide defers to SI conventions when a manuscript reports measurements, so learning SI formatting rules covers most of the unit questions a writer will face.

Formatting Rules for SI Units

• Always pair a numeral with the unit symbol, never spell out the number when a unit follows, as in 5 kg rather than five kg.
• Leave a single space between the numeral and the symbol, except for degree symbols attached to temperature scales, as in 25°C.
• Do not add a period after a unit symbol unless the symbol ends a sentence.
• Do not pluralize a unit symbol, so 10 kg is correct while 10 kgs is not.
• Keep unit symbols in the correct case, since case can change meaning, such as mW for milliwatt versus MW for megawatt.
• Use a centered dot, not a slash, when combining unit symbols, and use a negative exponent for division, as in  for meters per second.

Common SI Base and Derived Units

Quantity	Unit Name	Symbol	Example Usage
Length	meter	m	The rod measured 1.2 m in length
Mass	kilogram	kg	Each sample weighed 2 kg
Time	second	s	The reaction took 45 s
Temperature	kelvin	K	The chamber was held at 310 K
Amount of substance	mole	mol	The solution contained 0.5 mol
Volume	liter	L	Participants drank 1.5 L of water
Energy	joule	J	The device released 12 J of energy

Prefixes such as milli, centi, kilo, and micro attach directly to the unit symbol without a space or hyphen, as in mL, cm, kg, and micro g. Writers should also remember that SI recommends using a space, not a comma, to separate groups of three digits in long numbers, so 10 000 is preferred over 10,000 in strict SI usage, though many journals still accept the comma for readability.

Numbers in Chemical Compound Names

Chemical nomenclature follows rules set by the International Union of Pure and Applied Chemistry, known as IUPAC, rather than the general number rules used in prose. Numbers inside a chemical name identify the position of a substituent on a molecule or indicate how many times a group appears, and these numbers are never spelled out or treated as sentence-level numbers.

• Locants are numbers that mark the position of an atom or group on a carbon chain, such as the 2 in 2-chloropropane.
• Multiplying prefixes such as di, tri, and tetra indicate how many identical groups are present, as in 1,2-dichloroethane.
• Locants are separated from the rest of the name with a hyphen, and multiple locants are separated from each other with commas.
• Italics are reserved for locant letters such as N or O, while the numerals themselves stay in regular, non-italic type.
• When a compound name opens a sentence, style guides generally recommend rewording the sentence rather than capitalizing the locant, since capitalizing a locant number would misrepresent the chemical name.

Numbers in Gene and Protein Names

Gene and protein nomenclature is governed by organism-specific committees, such as the HUGO Gene Nomenclature Committee for human genes, and these conventions again take priority over general prose rules. Numbers within these names typically distinguish related family members or specific isoforms rather than functioning as ordinary quantities.

• Gene symbols are italicized in all species, such as TP53 for the human gene that encodes the p53 protein.
• Protein names that correspond to a gene are usually written in plain, non-italic type, such as p53, to visually distinguish the protein from the gene.
• Numbers within a gene symbol often indicate a family member, such as the 1 and 2 in BRCA1 and BRCA2.
• Superscript numbers or letters can indicate a specific allele or mutation in some organisms, such as mouse nomenclature.
• Writers should always confirm the current approved symbol through the relevant nomenclature database, since gene names are updated periodically and older names may become outdated.

Do All Style Guides Handle Number Ranges the Same Way?

No, style guides differ on how much of the second number to repeat in a range, though most use a hyphen without surrounding spaces to join the two numbers. APA and Vancouver generally repeat the full second number, while Chicago allows shortened forms for large numbers such as page ranges.

• APA writes full ranges such as 10-15 or 100-105, repeating every digit of the second number.
• Chicago permits shortened ranges for numbers over 100, such as 100-05 or 321-25, following its specific rules for dropping repeated digits.
• MLA typically repeats the full second number for clarity, especially in citations and page ranges.
• IEEE and Vancouver both favor the full, unabbreviated second number to avoid any ambiguity in technical and clinical reporting.
• A hyphen or en dash, not a comma or the word to, is the standard connector for a number range in running text, as in 5-10 participants.

Common Mistakes to Avoid in Academic Number Usage

Mixing Spelled-Out Numbers and Numerals in the Same Category

When several numbers in a sentence refer to the same kind of item, they should all follow the same format, even if one of them would normally be spelled out on its own.

• Incorrect: The study included three control groups and 12 treatment groups.
• Correct: The study included 3 control groups and 12 treatment groups.

Starting a Sentence with a Numeral

Every major style guide requires a sentence-opening number to be spelled out, or the sentence to be restructured so a numeral does not lead.

• Incorrect: 12 participants withdrew before the second session.
• Correct: Twelve participants withdrew before the second session.
• Correct: A total of 12 participants withdrew before the second session.

Pluralizing a Unit Symbol

Unit symbols are not words, so they never take a plural s, no matter how many units are being described.

• Incorrect: The samples weighed 10 kgs each.
• Correct: The samples weighed 10 kg each.

Adding a Period After a Unit Symbol Mid-Sentence

A period after a unit symbol is reserved for the end of a sentence. Adding one elsewhere can be misread as decimal punctuation or an abbreviation break.

• Incorrect: The dose was 5 mg. and was administered twice daily.
• Correct: The dose was 5 mg and was administered twice daily.

Using Outdated Gene or Protein Symbols

Nomenclature committees periodically revise approved symbols. Using a retired or informal name can confuse readers and database searches alike.

• Incorrect: The HER-2 gene was amplified in the tumor sample.
• Correct: The ERBB2 gene, formerly known as HER2, was amplified in the tumor sample.

Switching Between Unit Systems Without Conversion

Mixing metric and non-metric units without giving readers a conversion forces them to do the math themselves, which undermines clarity.

• Incorrect: The patient weighed 154 lb and received a dose of 2 mg/kg.
• Correct: The patient weighed 70 kg and received a dose of 2 mg/kg.

Frequently Asked Questions

Should I spell out numbers under 10 in APA format?

Yes, APA spells out numbers under 10 in general prose, but numerals are required even below 10 when the number precedes a unit of measurement, marks a statistic, or states an age, time, or date.

How do you write measurements in a scientific paper?

Measurements are always written as a numeral followed by a space and the correct unit symbol, such as 5 mL or 12 kg, with no period after the symbol and no plural s added to it.

Is it correct to start a sentence with a numeral?

No, nearly every major style guide, including APA, MLA, Chicago, IEEE, and Vancouver, requires a number that opens a sentence to be spelled out or the sentence to be reworded instead.

How do you format units of measurement in a research paper?

Units follow SI formatting rules: a numeral, a single space, then the unit symbol in the correct case, such as 25 mg, with no periods and no pluralization of the symbol.

Should gene names be italicized in a manuscript?

Yes, gene symbols are italicized in every species, such as TP53, while the corresponding protein name, such as p53, is written in plain, non-italic type.

What is the correct way to write a percentage in academic writing?

Percentages are always written with a numeral and the percent sign in scientific and technical styles, such as 8%, even when the number itself is below 10.

How do you write number ranges in APA versus Chicago style?

APA repeats every digit of the second number in a range, such as 100-105, while Chicago allows shortened forms for larger numbers, such as 100-05, under its own specific rules.

Do you use a space between a number and a unit symbol?

Yes, SI convention requires a single space between the numeral and the unit symbol, such as 10 kg, with the only common exception being the degree symbol in temperature notation.