How to Write Numbers in Formal Writing

Rules for writing numbers in formal documents vary by discipline and publisher, but a practical default reduces friction: spell out simple numbers in narrative text and use numerals for measurements, dates, percentages, statistics, and technical values. Confirm the specific thresholds and exceptions in the style manual you must follow, and apply a consistent approach throughout the document.

Core principles: the short rules you can apply now

Formal writing balances readability and precision. Use words for small, nontechnical numbers that flow in prose and numerals where precision or quick scanning matters. That principle resolves most decisions without memorizing every exception.

When in doubt, follow the required style guide and keep the document consistent. If no guide is mandated, adopt a clear rule at the start of the project and document it for coauthors or reviewers.

When to spell numbers out

• Use words for numbers that are part of ordinary narrative and not measurements: "three chapters", "several hundred people" when the exact figure is not central to the claim.
• Spell out numbers that begin a sentence. Better yet, rephrase the sentence to avoid starting with a numeral.
• Write out common fractions in words when they appear in running text: "one half" or "a third" in nontechnical contexts.

When to use numerals

• Use numerals for precise quantities, measurements, currency, dates, times, ages, percentages, and tabular or statistical data.
• Use numerals in technical writing or any context where readers will compare values quickly or perform calculations.
• Use numerals with units and abbreviations: "12 km", "5 mg", "3:00 p.m." — follow the linked unit and punctuation guidance for formatting details.

Style-guide differences and how to approach them

Major style guides do not agree on every threshold. Some explicitly set numeric cutoffs for when to spell out a number, others leave more discretion to the writer. Rather than memorizing every rule, determine which manual governs your work early and apply it consistently.

For a quick orientation to the main differences and where to look in the manuals, see the comparison page on APA vs Chicago vs MLA: Key Differences. If your document will be published, check the publisher's house style in addition to any academic guide.

Numbers with units and punctuation: common formatting rules

Formatting numbers with units introduces additional considerations: spacing, use of symbols, and punctuation. Technical fields often prefer SI units and specific spacing rules; humanities work may favor spelled-out units when readability benefits.

Consult specialized guidance for measurements and punctuation where necessary. See the linked resources on Formatting Measurements and Units and Punctuation Rules for Numbers for detail on symbols, spacing, and compound constructions.

Punctuation and thousands separators

• Use commas or thin spaces for thousands separators according to your guide - do not mix styles within one document.
• Decimal marks differ by locale - use a period or comma consistently and match the audience's expectation.
• Omit unnecessary punctuation when a number and unit are treated as one adjective: "a 5-kilometer run" versus "a run of 5 km".

Consistency and an editing checklist

Consistency is the single most important editorial goal after choosing a style. A short checklist during revision catches most problems quickly.

1. Identify the governing style guide and note any project-specific deviations.
2. Search for numerals at sentence starts and rephrase where possible.
3. Confirm uniform treatment of percentages, dates, times, and units across the document.
4. Check number formatting in lists, captions, and tables separately - these often follow different conventions.
5. Run a final pass for consistency using the linked proofreading checklist if available.

Use the linked Final Proofreading Checklist for Academic Papers to structure the final sweep through your manuscript. That checklist helps ensure you apply the same numeric rules in body text, figures, and references.

Step-by-step decision process you can follow

1. Ask: Is the number a precise measurement, statistic, date, time, age, money amount, or part of data? If yes, use numerals.
2. If the number is part of flowing prose and not central to meaning, prefer words for small values that read naturally.
3. If the style guide specifies a threshold or exception, apply that rule.
4. If multiple formats are possible, prioritize reader clarity and consistency within the document.
5. Document your chosen approach in a short "number style" note for coauthors and editors.

Worked examples and common mistakes

Concrete examples make the rules easier to apply. Below are sample sentences with recommended forms and explanations.

Raw: "20 students attended three lectures and 1 seminar during the week."

Recommended: "Twenty students attended three lectures and one seminar during the week." Explanation: Use words for numbers in narrative flow; small numbers spelled out improve readability.

Raw: "The survey found 5.4% increase in response rate compared to last year."

Recommended: "The survey found a 5.4% increase in response rate compared with last year." Explanation: Use numerals for percentages and place the percent in a context that highlights the statistic.

Common mistakes to avoid

• Inconsistent thresholds - alternating between words and numerals for the same kind of values in one document.
• Starting sentences with numerals - rephrase to avoid potentially awkward spelled-out numbers.
• Mixing locales - using a comma decimal in one section and a period in another without justification.
• Neglecting units or misplacing punctuation - such as "50 km/h." or inconsistent spacing "50km".

Closing: keep the rule, keep the reader in mind

There is no universal rule that fits every situation; the practical approach is to prioritize reader clarity. Use words for simple, narrative numbers and numerals for measurements, technical data, dates, percentages, and anything readers will compare or calculate. Confirm the exact rules in your required guide and run the editing checklist before submission to ensure consistency.

When you prepare a formal document, a short style note at the front - recording the numeric rules you used - saves time for reviewers and preserves the document's professional appearance.