31st is the correct spelling. 31th is incorrect and should never be used in formal or informal writing. The rule comes down to how English forms ordinal numbers: numbers ending in 1 typically take “-st,” not “-th,” and 31 follows that same pattern.
Quick Answer: 31st Is Correct
| Correct | 31st (thirty-first) |
| Incorrect | 31th |
| Why | Numbers ending in 1 (except 11) use the “-st” ending |
| Correct example | The meeting is scheduled for August 31st. |
| Incorrect example | The meeting is scheduled for August 31th. |
The confusion usually happens because so many other ordinal numbers end in “-th” (4th, 5th, 10th, 20th), and it’s easy to assume every number follows that pattern. It doesn’t.
Understanding Ordinal Numbers: The Basic Rule
Ordinal numbers show position or rank rather than quantity — first, second, third, and so on. English forms them using one of four endings: -st, -nd, -rd, or -th.
The -st, -nd, -rd, -th Pattern
The ending depends on the last digit of the number:
- Numbers ending in 1 → -st (1st, 21st, 31st, 41st)
- Numbers ending in 2 → -nd (2nd, 22nd, 32nd)
- Numbers ending in 3 → -rd (3rd, 23rd, 33rd)
- All other numbers → -th (4th, 15th, 30th, 100th)
Since 31 ends in the digit 1, it takes “-st,” giving you 31st.
Why “11th” Is the Exception That Confuses People
Here’s where the pattern breaks: 11, 12, and 13 all end in “-th” instead of following the digit-based rule. So it’s 11th, 12th, and 13th — not “11st” or “12nd.” This is simply an inherited quirk of English going back to its older number-naming system, and it’s the main reason people mistakenly assume “31th” should follow the same logic. It shouldn’t, because 31 doesn’t share the same root as eleven, twelve, or thirteen.
Why 31st Follows the Same Pattern as 1st, 21st, and 41st
Any number ending in 1 — except 11 — takes “-st.” That’s why the sequence looks like this:
1st → 21st → 31st → 41st → 51st
Once you see 31st sitting next to 21st and 41st, the pattern becomes obvious. Nobody writes “21th” or “41th,” so there’s no reason to write “31th” either.
Why Do People Write “31th” Instead of “31st”?
Most people write “31th” out of habit, not because they think it’s grammatically correct. A few common causes:
- Overgeneralizing the “-th” ending. Since most double-digit ordinals (20th, 40th, 50th) end in “-th,” it’s easy to apply that pattern to 31 by mistake.
- Typing speed and autocorrect gaps. Many spell-checkers catch obvious errors but don’t always flag “31th” in casual typing.
- Confusion with 11th. Because “eleventh” ends in “-th,” some writers assume any number ending in 1 might do the same.
None of these reasons make “31th” acceptable — it’s a spelling error, not a regional variation or alternate form.
How to Pronounce “31st” (Thirty-First)
“31st” is read aloud as thirty-first, pronounced roughly thur-tee-FURST, with the stress falling on the final syllable, “first.” It has three syllables: thir-ty-first. This pronunciation is consistent across American and British English — the spelling difference between US and UK English shows up in date order, not in how the word sounds.
Part of Speech: What Type of Word Is “31st”?
“31st” functions as an ordinal adjective when it modifies a noun (“the 31st floor”) and as a noun when it stands alone to refer to a date (“the meeting is on the 31st”). It does not have separate singular, plural, or verb forms, since ordinal numbers describe position rather than action or quantity.
| Use | Example |
|---|---|
| Adjective | She lives on the 31st floor. |
| Noun (referring to a date) | Rent is due on the 31st. |
31st in Dates: Correct Usage Examples
“31st” appears constantly in dates because only seven months (January, March, May, July, August, October, and December) have a 31st day.
Correct: The invoice is due on March 31st.
Correct: We’re closing the office on December 31st.
Incorrect: We’re closing the office on December 31th.
Writing Full Dates (American vs. British Style)
The two conventions differ in word order, not in the spelling of “31st” itself:
- American English: March 31st, 2026 (month–day–year)
- British English: the 31st of March, 2026 (day–month–year)
Both are correct within their own convention. What’s never correct in either style is swapping “31st” for “31th.”
Is 31st Spelled Differently in British and American English?
No. “31st” is spelled the same way in both British and American English. The only difference between the two varieties is how the full date is arranged and spoken, not the ordinal number itself.
Date Format Comparison Table
| Feature | American English | British English |
|---|---|---|
| Written date | 3/31/2026 | 31/3/2026 |
| Spoken date | March thirty-first | the thirty-first of March |
| Spelling of ordinal | 31st | 31st |
Ordinal Number Chart: 1st Through 31st
| Number | Ordinal | Number | Ordinal |
|---|---|---|---|
| 1 | 1st | 17 | 17th |
| 2 | 2nd | 18 | 18th |
| 3 | 3rd | 19 | 19th |
| 4 | 4th | 20 | 20th |
| 5 | 5th | 21 | 21st |
| 6 | 6th | 22 | 22nd |
| 7 | 7th | 23 | 23rd |
| 8 | 8th | 24 | 24th |
| 9 | 9th | 25 | 25th |
| 10 | 10th | 26 | 26th |
| 11 | 11th | 27 | 27th |
| 12 | 12th | 28 | 28th |
| 13 | 13th | 29 | 29th |
| 14 | 14th | 30 | 30th |
| 15 | 15th | 31 | 31st |
| 16 | 16th |
Common Mistakes with 31st
Mistake: The event falls on the 31th of the month.
Correction: The event falls on the 31st of the month.
Explanation: 31 ends in 1, so it takes “-st,” not “-th.”
Mistake: I’ll see you on Oct 31th.
Correction: I’ll see you on Oct 31st.
Explanation: Abbreviated dates follow the same ordinal rule as spelled-out ones.
Mistake: She was born on the 31rd of July.
Correction: She was born on the 31st of July.
Explanation: “-rd” belongs to numbers ending in 3, not 1 — a different but related mix-up.
31st vs. Similar Ordinal Mistakes (11th, 21st, 41st)
| Number | Correct Ordinal | Common Mistake |
|---|---|---|
| 11 | 11th | 11st |
| 21 | 21st | 21th |
| 31 | 31st | 31th |
| 41 | 41st | 41th |
Notice that 11 is the odd one out — it takes “-th” while 21, 31, and 41 all take “-st.” Keeping this short list in mind helps prevent both directions of the error.
How to Use “31st” in Everyday Writing
Emails
“Please submit the report by January 31st.”
News Writing
“The policy takes effect on July 31st, according to officials.”
Social Media
“New year, new me starting Dec 31st 🎉”
Formal Writing
“This agreement shall remain in effect through December 31st of the current fiscal year.”
Quick Practice: Test Yourself on Ordinal Numbers
- Choose the correct spelling: The contract ends on May 31st / 31th.
- Fill in the blank: October has ______ days, ending on the 31st. (30/31)
- Correct the sentence: “We leave for vacation on August 31th.”
- Choose the correct word form: 31 is a cardinal / ordinal number, while 31st is a(n) cardinal / ordinal number.
- Identify the correct ordinal for 21: 21st / 21th
- Correct the sentence: “Her birthday is on the 31rd of December.”
- Fill in the blank: Numbers ending in the digit 1 (except 11) take the ending ______.
- Choose between similar words: The deadline is the 31st / 31th of the month.
Frequently Asked Questions
Is “31st” or “31th” correct?
“31st” is the only correct spelling. “31th” is a common error that doesn’t follow standard English ordinal number rules.
Why isn’t it “31th” like “11th”?
“11th” is an exception in English ordinal numbers, along with 12th and 13th. Since 31 doesn’t share that root, it follows the regular rule for numbers ending in 1, which is “-st.”
How do you pronounce “31st”?
It’s pronounced “thirty-first,” with three syllables and the stress on the last syllable. The pronunciation doesn’t change between American and British English.
How do you write “31st” in a sentence?
Use it directly before or after the noun it modifies, such as “the 31st floor” or “May 31st.” It can also stand alone to refer to a date, as in “Rent is due on the 31st.”
Is “31st” used the same way in British and American English?
Yes, the spelling is identical in both. The difference lies in how full dates are ordered — American English uses month-day-year, while British English uses day-month-year.
What is the correct way to write a full date with “31st”?
In American English, write “March 31st, 2026.” In British English, write “31st March 2026” or “the 31st of March, 2026.”
Is “31st” a noun, adjective, or adverb?
It functions as an adjective when it modifies a noun (the 31st page) and as a noun when it refers to a date on its own (see you on the 31st).
What are the rules for ordinal number endings?
Numbers ending in 1 use “-st,” those ending in 2 use “-nd,” those ending in 3 use “-rd,” and everything else uses “-th.” The numbers 11, 12, and 13 are exceptions and always take “-th.”
Is “31th” ever acceptable in informal writing or texting?
No. Even in casual contexts like texting, “31th” is considered a spelling mistake rather than an accepted informal variant.
How do you write “31st” in Roman numerals or formal documents?
Formal documents typically still use “31st” for ordinal references. Roman numerals (XXXI) are generally reserved for cardinal contexts like numbering monarchs or major sections, not everyday ordinal dates.
What is the ordinal form of numbers similar to 31, like 21, 41, and 51?
All of them follow the same “-st” pattern: 21st, 41st, and 51st. Any number ending in 1 (other than 11) takes this ending.
The Bottom Line
31st is correct; 31th is not. The rule is simple once you separate 31 from the exception group of 11, 12, and 13: any number ending in the digit 1 takes “-st.” Whether you’re writing a date in an email, a report, or a text message, remembering the sequence 1st–21st–31st makes the correct spelling easy to recall.
Practice Answer Key
- 31st — 31 ends in 1, so it takes “-st.”
- 31 — October has 31 days.
- Corrected: “We leave for vacation on August 31st.” — “31th” is never correct; it should be “31st.”
- 31 is a cardinal number; 31st is an ordinal number.
- 21st — follows the same “-st” pattern as 31st.
- Corrected: “Her birthday is on the 31st of December.” — “31rd” incorrectly borrows the “-rd” ending used for numbers ending in 3.
- -st
- 31st
