31st or 31th? Discover the Correct Spelling Now

You are filling out a form, writing a birthday card, or setting a deadline, and you suddenly stop. Is it 31st or 31th? The number comes right after 30th, which ends in “th,” so the doubt makes sense. Many careful writers second-guess themselves at this exact spot.

This article gives you the answer right away and then explains the rule behind it. You will see how ordinal numbers are built, why 11th breaks the pattern, and how to write 31st in dates, emails, news copy, and formal writing. By the end, you will be able to spell any ordinal number with confidence and avoid the most common mistakes.

Quick Answer: 31st Is Correct

The correct form is 31st. The form “31th” is not standard English, and style guides do not accept it. If you write the number in words, the correct spelling is thirty-first.

The reason is simple. Ordinal numbers that end in 1 take the suffix “-st,” and 31 ends in 1. The same logic gives us 1st, 21st, 41st, and 101st.

Here are a few quick examples:

• Correct: Her 31st birthday is in May.
• Correct: The meeting is on October 31st.
• Incorrect: Her 31th birthday is in May.
• Incorrect: The meeting is on October 31th.

Understanding Ordinal Numbers: The Basic Rule

Ordinal numbers show position or order in a series. Cardinal numbers, such as 5, 12, or 31, tell you how many. Ordinal numbers, such as 5th, 12th, or 31st, tell you which one in a sequence.

In writing, an ordinal number can appear as numerals plus a short suffix (31st) or as words (thirty-first). The suffix is not random. It follows a clear pattern that depends on the last digit of the number.

The -st, -nd, -rd, -th Pattern

English uses four suffixes for ordinal numbers. The last digit decides which one you need:

• Numbers ending in 1 take -st: 1st, 21st, 31st, 101st
• Numbers ending in 2 take -nd: 2nd, 22nd, 32nd, 102nd
• Numbers ending in 3 take -rd: 3rd, 23rd, 33rd, 103rd
• All other numbers take -th: 4th, 10th, 25th, 30th

This is the ordinal numbers ending in 1 2 3 rule. If you remember only one thing, remember that 1, 2, and 3 have their own suffixes and everything else uses “th.”

Why “11th” Is the Exception That Confuses People

The numbers 11, 12, and 13 are the one real exception. They end in 1, 2, and 3, but they take “th” instead: 11th, 12th, and 13th. The reason is that the spoken words are eleventh, twelfth, and thirteenth, and these teen numbers follow their own older pattern.

This exception carries through to larger numbers that end in 11, 12, or 13. For example, 111th, 112th, and 113th all use “th.” The number 31 does not end in 11, so the exception does not apply. That is why 31st stays with “st.”

Why 31st Follows the Same Pattern as 1st, 21st, and 41st

The number 31 is built from “thirty” plus “one.” When you turn it into an ordinal, only the last part changes. Thirty stays the same, and “one” becomes “first,” which gives you thirty-first.

The numeral form works the same way. The word “first” is written 1st, so any number that ends in “-first” is written with “st.” Compare these pairs:

• twenty-first = 21st
• thirty-first = 31st
• forty-first = 41st
• fifty-first = 51st
• one hundred first = 101st

Once you see this pattern, the confusion fades. The number 31 is not a special case. It behaves exactly like every other number that ends in 1, except 11.

Why Do People Write “31th” Instead of “31st”?

Most people write “31th” because the number before it, 30th, ends in “th.” Numbers from 4 to 20 and every multiple of ten also take “th,” so “th” feels like the default ending. When the brain sees a number in the thirties, it often reaches for the most common suffix.

Another reason is speed. In fast typing or texting, people tend to add “th” without checking the last digit. The mistake also spreads because autocorrect does not always catch it, and some people copy what they have seen online.

A good habit is to look only at the last digit before you choose the suffix. If it is 1, use “st.” If it is 2, use “nd.” If it is 3, use “rd.” Otherwise, use “th,” unless the number ends in 11, 12, or 13.

How to Pronounce “31st” (Thirty-First)

You say 31st as “thirty-first.” The stress falls on the first syllable of “thirty” and again on “first.” In simple sound spelling, it sounds like THUR-tee-FURST.

Never say “thirty-oneth” or “thirty-one-th.” In speech, the ordinal ending only changes the last word, so “one” becomes “first.” The same is true for 21st (twenty-first) and 41st (forty-first).

Part of Speech: What Type of Word Is “31st”?

The term 31st is an ordinal number. Grammar books often treat ordinal numbers as a type of adjective, because they describe a noun by its position. In the phrase “the 31st floor,” the word 31st tells you which floor.

An ordinal number can also work as a noun when the noun is understood. For example, in “Her birthday is on the 31st,” the word “day” is left out, so 31st acts as a noun. Less often, an ordinal can work as an adverb, as in “She finished 31st.” In every case, the spelling stays the same.

31st in Dates: Correct Usage Examples

The 31st day of the month appears in seven months: January, March, May, July, August, October, and December. These are the months you will most often need to write “31st” or “31.”

Here are correct examples in sentences:

• The report is due on January 31st.
• Our lease ends on the 31st of July.
• She was born on March 31.
• The office closes early on December 31st.

Notice that you can write the date with or without the ordinal suffix. Both “October 31” and “October 31st” are acceptable in everyday writing. Formal style guides, however, often prefer the plain number.

Writing Full Dates (American vs. British Style)

American writers usually place the month first: October 31, 2026. A comma separates the day from the year. When the year is included, most style guides drop the ordinal suffix, so “October 31st, 2026” is less common in formal text.

British writers usually place the day first: 31 October 2026 or 31st October 2026. No comma is used between the month and the year. The ordinal suffix is more common in British writing, especially in letters and invitations.

Is 31st Spelled Differently in British and American English?

No. The spelling of 31st is the same in American and British English. Both varieties use “st” for numbers ending in 1, and both write the full word as “thirty-first.”

The differences appear only in how the date is arranged and punctuated. American English puts the month first and uses a comma before the year. British English puts the day first and often adds the ordinal suffix.

Date Format Comparison Table

The numeric formats can cause confusion, so it is safer to write the month in words when the reader may be from another country.

Ordinal Number Chart: 1st Through 31st

This chart shows the numeral form and the written form of each ordinal number from 1 to 31. It is a handy reference for how to write 31st in words and for checking nearby numbers.

Look at 30th and 31st side by side. The number 30 ends in 0, so it takes “th,” and its word form changes “y” to “ie” to become “thirtieth.” The number 31 ends in 1, so it takes “st.”

Common Mistakes with 31st

The most common error is writing “31th” instead of “31st.” Other mistakes appear often as well, and they are easy to fix once you know them.

• Writing “31th” or “31rd.” Only “31st” is correct.
• Leaving out the hyphen in words. Write “thirty-first,” not “thirty first.” Compound numbers from 21 to 99 take a hyphen, including ordinals.
• Putting the suffix on the same line as the number in a superscript without need. Some word processors raise the “st” automatically. This is fine, but plain text works just as well.
• Mixing styles in one document. If you write “October 31” in one place, avoid “October 31st” in another.
• Writing “31 st” with a space. The suffix attaches directly to the number: 31st.

31st vs. Similar Ordinal Mistakes (11th, 21st, 41st)

Writers who struggle with 31st often make the same mistake with other numbers. The following pairs show the correct and incorrect forms side by side:

• 11th is correct. “11st” is wrong because 11 is an exception.
• 21st is correct. “21th” is wrong.
• 31st is correct. “31th” is wrong.
• 41st is correct. “41th” is wrong.
• 111th is correct. “111st” is wrong because it ends in 11.

The pattern is easy to remember. Check the last two digits first. If they are 11, 12, or 13, use “th.” If not, look at the last digit and apply the 1, 2, 3 rule.

How to Use “31st” in Everyday Writing

The right format depends on where you are writing. The spelling of 31st does not change, but the style can. Here is how to handle the most common situations.

Emails

In emails, both “31st” and “31” work well. Choose the one that reads naturally and keep it consistent. For example: “Please send the files by October 31st.” Or: “Our next meeting is on the 31st at 10 a.m.”

News Writing

News style, such as the Associated Press style, usually drops the ordinal in dates. A headline or story would say “Oct. 31” rather than “October 31st.” Ordinals are still used for rankings, such as “the 31st annual festival.”

Social Media

On social media, “31st” is perfectly fine and often feels more casual. Posts like “Happy 31st birthday!” or “See you on the 31st!” are common and clear. Keep the spelling correct, since small errors are easy to notice in short posts.

Formal Writing

In formal writing, such as academic papers, legal documents, and invitations, the rules are stricter. Many style guides, including the Chicago Manual of Style, prefer spelling out ordinals under one hundred in running text. In that case, write “the thirty-first anniversary” instead of “the 31st anniversary.”

Which Months Have a 31st?

Knowing which months have a 31st day helps you avoid writing impossible dates. Seven months have 31 days:

• January
• March
• May
• July
• August
• October
• December

April, June, September, and November have only 30 days. February has 28 days, or 29 in a leap year. A date such as “April 31st” does not exist, so it should never appear in a schedule or contract.

Quick Practice: Test Yourself on Ordinal Numbers

Try these short questions. Choose the correct form for each one. The answers are listed below.

1. Her (31st / 31th) birthday party is on Saturday.
2. The store opens on the (21st / 21th) of June.
3. He finished in (11st / 11th) place.
4. The (41st / 41th) edition of the book is out now.
5. Write the number 31 as an ordinal in words.

Answers:

1. 31st
2. 21st
3. 11th
4. 41st
5. thirty-first

If you got four or five correct, you understand the rule well. If you missed one, review the 1, 2, 3 pattern and the exception for 11, 12, and 13.

Frequently Asked Questions

Do we say 31th or 31st?

We say and write 31st. The form 31th is incorrect because numbers ending in 1, except 11, take the suffix “st.”

Is it 31st or 31th birthday?

It is a 31st birthday. Say “thirty-first birthday” aloud, and write the numeral as 31st.

Is it 31th or 31st October?

The correct form is 31st October in British style. In American style, write October 31 or October 31st.

Is it December 31st or 31th?

Write December 31st or simply December 31. The form “December 31th” is not correct.

How do you write 31st in words?

Write it as “thirty-first,” with a hyphen between the two parts. Both words are lowercase unless they start a sentence.

Is 31st a noun or an adjective?

It is an ordinal number. It usually works as an adjective, as in “the 31st floor,” but can act as a noun.

Final Thoughts

When you wonder about 31st or 31th, the answer is always 31st. Numbers that end in 1, except 11, take “st,” and 31 follows that rule just like 1st, 21st, and 41st. In other words, the correct form is thirty-first. The mistake “31th” usually comes from the habit of adding “th” after 30th.

Remember the simple check: look at the last digit, then watch for 11, 12, and 13. In dates, American style puts the month first and often drops the suffix, while British style puts the day first and often keeps it. Whether you are writing an email, a news story, or a formal letter, you can now use 31st with confidence and spot the same error in other ordinal numbers.

Ethan Whitmore

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.