Plurals and Punctuation of Numbers in Academic and Scientific Writing

Plurals and Punctuation of Numbers in Academic & Scientific English

1. Why Number Formatting Matters

Numbers appear everywhere in academic and scientific writing: in tables, figures, equations, measurements, dates, statistics, and everyday prose. Because they are so common, even small inconsistencies can be visually jarring or misleading. A stray apostrophe, an unexplained comma, or an incorrect space can distract examiners and reviewers or, worse, introduce ambiguity into your results and calculations.

Fortunately, most conventions are straightforward once you understand the underlying principles. This article focuses on two key aspects:

• plurals of numbers and quantity words, and
• punctuation and spacing around numbers and units.

The aim is to help you apply consistent, professional formatting in theses, dissertations, journal articles, and technical reports – and to give you the language needed to interpret journal or university guidelines when they are brief or incomplete.

2. Plural Forms of Numbers Written as Words

When numbers are written out as words and used as nouns (for example, when you refer to the numbers themselves or to decades), they take plurals in the same way as other English nouns. Apostrophes are not used to form these plurals.

2.1 Basic number words

Compare:

• Correct: The students were all at sixes and sevens.
• Incorrect: The students were all at six’s and seven’s.

Here, sixes and sevens are simple plurals formed by adding -es to the number words. No apostrophes are needed or appropriate.

2.2 Decades and similar forms

The same principle applies when referring to decades:

• Correct: She specialises in literature from the 1960s and 1970s.
• Incorrect: She specialises in literature from the 1960’s and 1970’s.

In formal writing, the plural of a decade is usually formed simply by adding an s to the year (1960s, 1990s). Apostrophes (1960’s) were more common in older styles, but are now often considered incorrect or outdated in academic contexts.

3. “Dozen”, “Hundred”, “Thousand” and Larger Quantities

Certain quantity words behave a little differently. Words such as dozen, hundred, thousand, million, billion and trillion are usually kept in the singular when they are directly preceded by a specific number:

• four thousand participants
• eight million records
• three hundred samples

However, when the quantity is indefinite, these words typically take plural forms:

• dozens of icebergs were visible from the coast
• hundreds of birds migrated through the area
• millions of people rely on this technology

Remember:

• definite numeric expression → no plural “s” (four thousand, seven million);
• indefinite quantity expression → plural “s” (hundreds, thousands, millions).

4. Abbreviated Measures and Units of Measurement

Scientific and technical writing often uses abbreviated units of measurement (cm, kg, mL, °C, etc.). These abbreviations follow different rules from ordinary nouns.

4.1 No plural “s” on abbreviations

When units are written in their abbreviated form and used alongside numerals, they do not change form for plural:

• Correct: 44 cm, 12 kg, 20 °C
• Incorrect: 44 cms, 12 kgs, 20 °Cs

In words, you would write “44 centimetres”, “12 kilograms” and “20 degrees Celsius,” but in abbreviated form the unit remains the same regardless of the number.

4.2 Spacing between number and unit

In most scientific styles, there is a space between the numeral and the unit abbreviation:

• 44 cm
• 12 kg
• 5 mL
• 20 °C

This also applies to SI units in general. The space is part of the formal layout and improves readability, especially in tables and figures. An exception is made when numbers are part of codes or labels rather than measurements; we will return to this below.

4.3 When numbers appear with other letters

When numerals are associated with alphabetic labels or codes rather than units of measure, there is usually no space between the number and the letter(s):

• Question 2a
• Figure 3b
• pp. 33ff

Here, the number and letter form a single label, not a measurement expression.

5. Compound Numbers and Fractions Written as Words

Compound numbers (twenty-one, eighty-seven, one hundred and thirty-four) and fractional expressions also have standard formatting conventions in formal English.

5.1 Hyphenation in compound numbers

When writing compound numbers as words, hyphenate the elements from twenty-one to ninety-nine when they function as single units:

• twenty-four participants completed the survey
• seventy-ninth percentile
• thirty-three-year-old patients

Beyond ninety-nine, hyphenation typically applies only to the last two digits:

• one hundred and twenty-five (commonly without hyphen between “hundred” and “and”)
• three hundred and sixty-four

Stylistic details (such as whether to write “one hundred and five” or “one hundred five”) depend on regional and style preferences, so check the relevant style guide, but the hyphenation of compound numbers is widely accepted.

5.2 Fractions as words

Fractions written out in words are usually hyphenated, especially when they function as adjectives:

• one-quarter of the sample
• a two-thirds majority
• three-fifths of the participants

However, fractions can be left open when the focus is on the parts themselves rather than the compound phrase, or when clarity and rhythm benefit from separate words:

• He gave one third of the pie to his friend and another third to his brother.

In mathematical or technical contexts, numeric forms (1/3, 2/5) are often preferred, but when fractions appear in running text, consistent hyphenation helps readers recognise them quickly.

6. Grouping Digits in Large Numbers and Decimals

Another important aspect of number formatting is how you group digits in large numerals and decimals. The main goal is to make numbers easy to read at a glance and to avoid miscounting the zeros.

6.1 Commas or spaces in large numbers

In many English-language contexts, large numbers are written with commas separating groups of three digits from the right:

• 4,578
• 67,512
• 1,234,567

In technical and scientific writing – especially where international conventions are followed – spaces are often used instead of commas to separate these groups:

• 4 578
• 67 512
• 1 234 567

Using spaces helps avoid confusion in contexts where commas may be used as decimal separators. Some styles omit both commas and spaces in four-digit numbers (e.g. 4578), but add grouping for larger numbers. The key is to follow your discipline’s standard and be consistent throughout the document.

6.2 Grouping digits after the decimal point

The same grouping logic can be applied to digits after the decimal point, particularly in technical reports:

• 17.239 8
• 5.349 65

Here, spaces are inserted after every three digits following the decimal, to aid readability. Where there are only a few digits, grouping is usually not applied (for example, 17.2398). Some style guides discourage grouping in decimal parts altogether. Again, check the conventions and follow them consistently.

7. Additional Common Issues with Numbers

Beyond the main points above, several other details frequently cause problems in academic writing.

7.1 Apostrophes with numbers

Apostrophes should not be used to form plurals of numerals or abbreviated quantities:

• Correct: three 7s in a row
• Incorrect: three 7’s in a row
• Correct: the 1990s saw rapid growth
• Incorrect: the 1990’s saw rapid growth

Apostrophes with numbers are reserved for possessives, which are relatively rare in formal numerical expressions.

7.2 Ranges and hyphens

Number ranges in text are often indicated with a hyphen or en dash (depending on the style), without spaces in most styles:

• pages 35–47
• ages 18–25
• temperatures of 10–15 °C

Some styles prefer “from X to Y” constructions instead of using a dash. Avoid doubling up (“from 10–15 °C”) – choose either a dash or “from…to,” but not both.

7.3 Spelling out vs. using numerals

Decisions about when to spell out numbers (one, two, three) and when to use numerals (1, 2, 3) vary by style guide. A common pattern in academic prose is:

• spell out numbers from one to nine in running text;
• use numerals for 10 and above, and for anything involving measurement, data, or statistics (3 cm, 7 kg, 2.5%, 6 participants).

However, many journals and disciplines adopt different rules, for example using numerals for all numbers in scientific writing. Always follow the relevant guidelines and be consistent.

8. Accuracy, Consistency, and Final Checks

The choices involved in expressing numbers effectively can be surprisingly complex, especially in documents that combine narrative, mathematics, tables, and figures. Yet accuracy and consistency are essential for clear communication and reliable analysis.

Before submitting your work:

• review your journal’s or university’s style guide for rules on numbers and units;
• check that hyphens, commas, and spaces in large numbers are used systematically;
• ensure that plurals of number words and units are formed correctly (no stray apostrophes, no plural “s” on abbreviations);
• verify that fractions, ranges, and percentages follow a single, consistent style.

Because numerical details are easy to overlook when you have been immersed in the content of your research, a final proofread focused specifically on numbers can be invaluable.

Many authors also seek help from professional academic proofreaders, who are familiar with number conventions across different disciplines and style guides. Whether you are preparing a thesis, research article, or technical report, careful attention to the plurals and punctuation of numbers will help your readers focus on your findings instead of being distracted by avoidable formatting errors.