You are typing a date on an invoice, a party invitation, or a school form, and your fingers stop. Should it be 31st or 31th? The number ends in 1, but most numbers after 3 take “th,” so the doubt is understandable, and plenty of careful writers pause at this exact spot.
This article gives you the answer right away, then shows why the rule works. You will see how ordinal suffixes are formed, how to write 31st in dates and sentences, how American and British styles differ, and which mistakes to avoid. Every example uses standard English, so you can copy the patterns with confidence in emails, essays, and formal documents.
31st or 31th: Quick Answer
The correct form is 31st. The spelling “31th” is a mistake in both American and British English, and you will not find it in any standard dictionary or style guide. The full word is “thirty-first,” and the suffix “st” comes from the last two letters of “first.”
The reason is simple. English builds an ordinal number from the last word in the spoken number, and the numeral copies the ending of that word. Because 31 is read as “thirty-first,” the written form is 31 plus “st.”
Ordinal Suffix Rule at a Glance
The ordinal suffix rules are short enough to memorize. Look at the last digit of the number, with one exception for the teens:
- Numbers ending in 1 take st: 1st, 21st, 31st, 41st, 101st
- Numbers ending in 2 take nd: 2nd, 22nd, 32nd, 42nd
- Numbers ending in 3 take rd: 3rd, 23rd, 33rd, 43rd
- Numbers ending in 4 through 9, and 0, take th: 4th, 15th, 30th, 50th
- Numbers ending in 11, 12, or 13 take th: 11th, 12th, 13th, 111th
Since 31 ends in 1 and is not part of the 11 to 13 group, it takes “st.” Once you apply this rule, the question of whether 31st or 31th correctly answers itself.
31st or 31th in Dates: Correct Usage Examples
Dates are where most people meet this number, because 31st in dates appears on seven months of the calendar: January, March, May, July, August, October, and December. In every one of those months, the last day is written with “st.” You can write it with the month first or the day first, depending on your region and style.
Here are correct examples:
- The deadline is December 31st.
- Your lease ends on March 31st.
- The store closes on the 31st of July.
- Registration runs through October 31st.
Notice that “th” never appears in any of these sentences. If you see 31th spelling in a draft, replace it with “st” every time. This holds for invoices, calendars, contracts, and social media posts alike.
The Origin of 31st or 31th
English ordinals grew out of spoken words, and the written shorthand simply mirrors them. The words “first,” “second,” and “third” are irregular, which is why their abbreviations end in “st,” “nd,” and “rd.” Most other ordinals add “th” to the number word, as in “fourth,” “ninth,” and “twentieth.”
When a larger number ends in 1, 2, or 3, it borrows the irregular ending. Twenty-one becomes “twenty-first,” thirty-one becomes “thirty-first,” and so on. That is why 31 behaves like 1 and not like 4 through 9.
People sometimes wonder about thirty-first or thirty-oneth. The second version does not exist. English never forms an ordinal by adding “th” to the word “one,” so “thirty-oneth” and “31th” are both errors, rooted in the same wrong guess that every ordinal should end in “th.”
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British English vs American English Spelling
The good news is that there is no spelling difference here. Both American and British English use 31st. If you were searching for 31st or 31th in American English or 31st or 31th in British English, the answer is identical: “st” in both.
The real difference is how the date is arranged. American writers usually put the month first, as in October 31, while British writers put the day first, as in 31 October. The ordinal suffix is optional in both styles, though it is more common in speech and informal writing.
Comparison Table
| Style | Common Format | Example | Uses “31th”? |
| American, standard | Month Day, Year | October 31, 2026 | No |
| American, with suffix | Month Day(st) | October 31st | No |
| British, standard | Day Month Year | 31 October 2026 | No |
| British, with suffix | Day(st) Month | 31st October | No |
| Written out | the thirty-first of Month | the thirty-first of October | No |
The pattern is clear. Whatever the regional format, the suffix on 31 is always “st.”
Which Form Should You Use?
Use 31st whenever you want an ordinal. If you prefer to avoid the suffix, you can write the plain numeral, as in “October 31,” which is the standard style in American news writing and many business documents. Both choices are correct, and neither involves “th.”
For formal writing, such as legal documents, invitations, and academic papers, you may spell the number out: “the thirty-first of December.” Many style guides also suggest spelling out ordinals below 10 in running text, but numerals are fine for 31 in dates and lists. When in doubt, follow the style guide for your school, employer, or publisher, and keep the suffix consistent within one document.
Common Mistakes with 31st or 31th
Most errors with this number come from habit, not from lack of knowledge. A few patterns appear again and again, and knowing them makes them easy to catch while proofreading.
1. Assuming Every Ordinal Ends in “-th”
Because most ordinals end in “th,” many writers apply it to everything. That works for 4th, 5th, and 30th, but not for numbers ending in 1, 2, or 3. A quick check of the last digit fixes this: 1 means “st,” 2 means “nd,” 3 means “rd.”
2. Typing “31th” from Muscle Memory
After typing 30th, your fingers may repeat the same suffix on the next number. Autocorrect does not always catch it, since “31th” is not a word. Reread dates slowly, especially at the end of a month, when 30th and 31st often appear together.
3. Confusing 31st with 30th
The two numbers sit side by side, but they take different endings. The 30th ends in “th” because the spoken form is “thirtieth.” The 31st ends in “st” because the spoken form is “thirty-first.” Compare 30th vs 31st this way: the last word is “thirtieth” in one case and “first” in the other.
4. Adding a Space Before the Suffix
Write 31st, not “31 st.” The suffix attaches directly to the number with no space. Some writers also raise the letters, as in 31<sup>st</sup>, which is acceptable in print, but plain text without superscript is the safer choice for emails and web pages.
31st or 31th Ordinal Numbers
Ordinal numbers show position or order, such as first place, second floor, or 31st day. They differ from cardinal numbers, which show quantity, such as one, two, or 31. In ordinal numbers in English, the suffix tells the reader that the number is a position in a sequence.
The rule also applies to other numbers in the same family. Compare these correctly written examples:
- 21st, 31st, 41st, 51st, 101st
- 22nd, 32nd, 42nd
- 23rd, 33rd, 43rd
- 11th, 12th, 13th, 111th, 112th, 113th
The teens are the main exception. Even though 11 ends in 1, 12 ends in 2, and 13 ends in 3, they take “th” because the spoken forms are “eleventh,” “twelfth,” and “thirteenth.” Numbers like 21 and 31 are not teens, so they follow the regular pattern.
31st or 31th in Everyday Examples
Seeing the correct form in context makes it easier to remember. The examples below cover the most common situations where people search for the right spelling.
October 31st or 31th
The correct form is October 31st. This is the date of Halloween in the United States, so you will see it on posters, school calendars, and store signs. A sentence might read, “The costume party starts at 7 p.m. on October 31st.”
August 31st or 31th
Write August 31st. It is the last day of the month and often marks a deadline for summer programs, rental agreements, and fiscal reports. For example, “Applications must be submitted by August 31st.”
31st or 31th Years Old
Neither is correct for age. You say “31 years old” or “I am 31.” The ordinal appears only when you refer to the year of life, as in “He is in his 31st year.” Compare these examples:
- Correct: She is 31 years old.
- Correct: He is in his 31st year of teaching.
- Incorrect: She is 31st years old.
The same logic applies to a 31st birthday and a 31st wedding anniversary, where the ordinal describes which one it is. “Her 31st birthday is on Saturday” is correct, while “31th birthday” is not.
31st or 31th March
Write 31st March in British style or March 31st in American style. For example, “The financial year ends on 31st March.” Many American readers will expect “March 31” instead, so match your audience.
31st or 31th of July
The correct phrase is 31st of July, or “July 31st” in American style. In speech, people often say “the thirty-first of July” or “July thirty-first.” Both are natural and correct.
31st or 31th May
Use 31st May in British style and “May 31st” in American style. The suffix stays the same, so “31th May” should be corrected. A proper sentence is, “Entries close on 31st May.”
32nd or 31th
Neither of these is the right comparison, because 31 and 32 take different endings. The correct forms are 31st and 32nd. If you are comparing 32nd vs 31st, the logic is simple: 31 ends in 1 and takes “st,” while 32 ends in 2 and takes “nd.”
31st Pronunciation
Say it as “thirty-first.” The 31st pronunciation places stress on both “thirty” and “first,” and the “t” in “first” is often light in fast speech. You never say “thirty-oneth,” which is another reminder that “th” does not belong on this number.
Final Thoughts
The right choice is 31st. English forms ordinal numbers from the final word of the spoken number, and “thirty-first” ends in “first,” so the written form ends in “st.” Numbers ending in 1, other than 11, always follow this rule, from 21st and 31st to 101st.
Remember the short checklist: 1 takes “st,” 2 takes “nd,” 3 takes “rd,” and almost everything else takes “th,” including 30. Keep the suffix attached to the number, match your regional date order, and spell out the number only when your style guide asks for it. With these habits, you can write dates and rankings correctly every time.
Frequently Asked Questions
Is it 31th or 31st October?
It is 31st October. The suffix “st” is correct because the spoken form is “thirty-first.” American writers usually prefer “October 31.”
Is it December 31st or 31th?
It is December 31st. Numbers ending in 1, except 11, take “st,” so “31th” is always a spelling mistake.
Is it 31th birthday or 31st birthday?
Use 31st birthday. The ordinal matches the spoken phrase “thirty-first birthday.” Nobody says “thirty-oneth,” so “31th” is not valid.
Is it 30th or 31st?
Both are correct for different numbers. Write 30th for thirty and 31st for thirty-one, because the endings follow “thirtieth” and “thirty-first.”
Is it 31rd or 31st?
The correct form is 31st. The suffix “rd” is only for numbers ending in 3, such as 3rd, 23rd, and 33rd.
Are 11th, 12th, and 13th exceptions to the rule?
Yes. Despite ending in 1, 2, and 3, they take “th,” as in 11th, 12th, and 13th. Numbers like 21st and 31st follow the regular rule.
Ethan Whitmore is an English language writer who enjoys making grammar, word meanings, and modern slang simple and easy to understand. His content focuses on clear explanations, practical examples, and everyday English usage.