Writing numbers correctly in professional documents comes down to a few straightforward rules that prevent confusion. If you're wondering how to spell 30, the correct written form is thirty. Spelling numbers out properly is essential for legal documents, financial reports, academic papers, and formal correspondence.
While typing numerical digits like 30 is common in casual contexts, style guides often require spelling out numbers under certain thresholds. Knowing when and how to write thirty—along with its ordinal form thirtieth—keeps your prose polished and precise.
Understanding the Standard Spelling of 30
The standard word for the number 30 is thirty. Many writers accidentally misspell it as thirtety or thirtee, letting the pronunciation of thirteen bleed into the decade form.
The breakdown of the spelling is straightforward:
- Root number: three becomes the base thir- prefix.
- Decade suffix: The suffix -ty denotes tens (similar to fourty—which is also a misspelling, actually forty—fifty, and sixty).
When you combine thir- and -ty, you get thirty. It has an 'h' right after the 't', and only one 'e'. Keeping this visual pattern in mind helps you avoid the common typo thirtee.
When to Spell Out Numbers vs. Using Digits
Different publishing style guides enforce specific conventions for when to write out numbers as words and when to use Arabic numerals. Understanding these rules ensures your professional writing meets industry standards.
The General Rule for Small Numbers
Most mainstream style guides, including the Associated Press (AP) Stylebook and the Chicago Manual of Style, recommend spelling out whole numbers below 10 or 100, depending on the context.
- AP Style: Spell out numbers from zero through nine, and use figures for 10 and above. Under this rule, you would write 30 as a digit in news reporting and journalistic writing.
- Chicago Style: Spell out whole numbers from zero through one hundred in general prose, reserving numerals for percentages, measurements, and figures above one hundred. Under this rule, you must spell out thirty in standard essays, books, and formal non-fiction.
Exceptions Requiring Digits
Even under style guides that favor spelling out numbers, certain contexts demand Arabic numerals for clarity and quick scanning:
- Percentages and Statistics: Always write 30% rather than thirty percent in technical or scientific reports.
- Measurements and Money: Use digits for units like 30 miles, 30 pounds, or $30.
- Addresses and Dates: Write 30 Elm Street or November 30.
- Beginning of a Sentence: If a sentence must start with the number, you must spell it out—Thirty employees attended the seminar—to avoid starting a sentence with an awkward digit.
Spelling Variations: Thirty vs. Thirtieth
Writing the cardinal number is only part of the challenge. You also need to know how to spell its ordinal counterpart when denoting position or sequence.
- Cardinal number (amount): thirty (e.g., There are thirty days in November.)
- Ordinal number (position): thirtieth (e.g., She celebrated her thirtieth birthday.)
Notice that thirtieth drops the final 'y' of thirty and adds -ieth. Misspellings like thirtyth are common errors that undermine professional credibility.
Compound Numbers Involving 30
When 30 is combined with single digits from 1 to 9, you create compound numbers. Proper hyphenation is critical here.
- 31: thirty-one
- 32: thirty-two
- 35: thirty-five
- 39: thirty-nine
Always include a hyphen between the tens place and the units place for compound numbers from twenty-one through ninety-nine. Omitting the hyphen is a frequent grammatical oversight in business writing.
Common Mistakes to Avoid
Even experienced writers occasionally trip up on decade spellings. Watch out for these pitfalls:
- Dropping the 'h': Writing tirty instead of thirty is a phonetic spelling error.
- Confusing with Thirteen: Mixing up thirteen (13) and thirty (30) changes the meaning of a sentence entirely. Double-check your drafts when dealing with financial quantities or ages.
- Incorrect Hyphenation: Writing thirty five without a hyphen is technically incorrect in formal grammar when the phrase acts as a modifier or compound number.
For deeper insights into standard English grammar and style rules, you can consult resources like the Purdue Online Writing Lab (OWL) or official references from Merriam-Webster.
Frequently Asked Questions
Is it ever acceptable to write 30 as a digit in formal essays?
It depends entirely on the style guide you follow. Academic writing following the Chicago Manual of Style requires spelling out thirty in most narrative contexts, whereas business or technical writing often prefers the digit 30 for brevity.
How do you write 30 in a formal legal contract?
Legal documents typically spell out the number and follow it with the digit in parentheses to eliminate any ambiguity. You would write thirty (30) days notice.
Why is forty spelled without a u, but thirty keeps the i?
English spelling historical shifts altered words independently over centuries. While forty lost its internal 'u' from the older root four, thirty retained the thir- prefix derived from three.
How do I write fractions or decimals involving 30?
Decimals and complex fractions are almost always written with digits for clarity, such as 30.5 or 30 1/2, rather than spelling them out word-for-word.
