Ever typed a date, paused, and thought “wait, is it 31st or 31th”? You’re not alone. Thousands of people search this exact question every month, and honestly, the confusion makes sense. English ordinal numbers don’t always follow a pattern that feels obvious. This guide clears it up for good, with real examples you’ll actually use.
31st or 31th: Quick Answer
31st is correct. 31th is not a real word in standard English, and you’ll never see it in a dictionary, a newspaper, or a properly written email.
The rule comes down to the last digit of the number. Since 31 ends in 1, it takes the -st suffix, just like 1st and 21st. 31th might look logical if you’re guessing based on other ordinal number endings, but English grammar doesn’t work that way here. Once you know this one rule, you’ll never second guess it again.
31st or 31th in Dates: Correct Usage Examples
Dates are where this mistake shows up the most. People write invitations, emails, and calendar entries every single day, and a typo like 31th can make a message look rushed or careless.
Here are some correct usage examples you can copy directly:
- December 31st marks the last day of the year.
- March 31st is often a fiscal quarter deadline.
- October 31st is Halloween.
- January 31st wraps up the first month.
- May 31st falls near the end of spring.
Notice a pattern? No matter the month, 31st stays the same. The month changes, the date format doesn’t.
The Origin of 31st or 31th
Where did 31st even come from? The story goes back to Old English counting systems, where numbers had spoken forms long before anyone standardized spelling. Ordinal numbers like first, second, and third grew out of natural speech patterns, not strict rules someone wrote down in a textbook.
As English language development moved forward, certain endings stuck because of how they sounded when spoken aloud. Thirty-first naturally ends in a sound close to “st,” so the written form followed the spoken form. That’s really the heart of it. Pronunciation shaped spelling more than any formal grammar committee did.
This history explains why the rule feels a bit random at first glance. It wasn’t designed on a whiteboard. It grew organically, the way most of the English language did.
British English vs American English Spelling
Here’s some good news if you’ve been worrying about regional variation: this one’s universal. Both British English and American English use 31st, without exception.
Unlike words such as “colour” versus “color,” ordinal number spelling doesn’t split by region. Whether you’re writing a letter in London or an email in Los Angeles, the same ordinal rules apply. This is one of the few corners of English where everyone agrees.
Comparison Table
| Number | British English | American English |
|---|---|---|
| 1 | 1st | 1st |
| 3 | 3rd | 3rd |
| 21 | 21st | 21st |
| 31 | 31st | 31st |
| 41 | 41st | 41st |
As you can see, there’s no regional difference here at all. It’s a universal rule across every English speaking country.
Which Form Should You Use?
Always use 31st. There’s no context, formal or informal, where 31th becomes acceptable. Not in professional writing, not in a casual text to a friend, not even in slang.
Some writers assume informal settings allow more flexibility, but that’s not really true here. 31st isn’t a stylistic choice, it’s simply the correct form. Think of it like spelling “receive” correctly. You wouldn’t write “recieve” just because you’re texting casually. The same logic applies to ordinal numbers.
If you’re ever unsure while writing quickly, a good trick is to say the number out loud. “Thirty-first” sounds nothing like “thirty-th,” and your ear will usually catch the mistake before your fingers do.
Common Mistakes with 31st or 31th
Let’s talk about where things go wrong. Most grammar mistakes with ordinal numbers come from overgeneralizing one rule across every number, which sounds reasonable but backfires quickly.
Watch out for these frequent slip ups:
- Writing 31th instead of 31st
- Assuming all numbers ending in 1 get -st, then forgetting exceptions like 11th
- Adding a space between number and suffix, like “31 st” or “31 th”
- Mixing up 21th or 41th with their correct forms, 21st and 41st
- Confusing 31rd with 31st, which mashes up two different suffixes
The 11, 12, and 13 exceptions trip up a lot of people too. You’d expect 11 to take -st since it ends in 1, but it doesn’t. It’s 11th, 12th, and 13th, no matter what. These three numbers simply ignore the last digit rule, and there’s no clever trick to explain why beyond “that’s just how English decided to do it.”
31st or 31th: Common Search Questions Answered
People type all kinds of variations into Google when they’re unsure. Let’s knock out the most common ones directly.
Is 31th correct? No, it isn’t. There’s no context where this spelling holds up.
Is it 31rd or 31st? It’s 31st. 31rd doesn’t exist in English at all, it’s a mix up between the -rd ending (used for 3rd) and the correct -st ending for 31.
How do you write 31th in words? You don’t, because the correct written form is “thirty-first,” never “thirty-th.”
Should I write 31st or just 31? That depends on context. Dates like “December 31” are common in American date formats, while “31st December” is more typical in British style. Both drop or keep the suffix depending on convention, but when you do use a suffix, it’s always -st.
31st or 31th in Everyday Examples
Grammar rules matter more when you see them in action. Here’s how 31st shows up across different writing contexts you deal with regularly.
Emails
Professional emails demand accuracy, since small errors can shape how people perceive your attention to detail. “The deadline for submissions is March 31st” reads clean and confident. Swap in 31th, and it instantly looks like a typo that slipped past proofreading.
News Writing
Journalists follow strict style guides, and news writing almost universally sticks to 31st. Headlines like “Company Reports Earnings on October 31st” show up constantly, reinforcing just how standard this spelling really is across publications.
Social Media
Even in social media posts, where grammar tends to loosen up, 31st stays intact. You’ll rarely spot 31th trending anywhere, because native speakers instinctively know it looks wrong, even if they can’t explain the rule.
Formal Writing
Formal writing, whether it’s a contract, an application, or an official document, leaves zero room for error. 31st is the only acceptable version here, and getting it wrong in a legal or business document can raise unnecessary questions about carelessness.
31st or 31th: Google Trends and Usage Data
Search behavior tells an interesting story. Google Trends data consistently shows searches like “is 31th correct” and “31st or 31th” spiking throughout the year, especially around birthdays, New Year’s Eve, and end of month deadlines.
This pattern suggests people aren’t confused about the concept of ordinal numbers overall. They’re specifically unsure about this one number. Compare that to searches for something like “4th or 4th,” which barely exist, since numbers like 4 rarely cause hesitation. 31, sitting right at the edge of a month and carrying that unusual “thirty” prefix, seems to trigger more doubt than smaller numbers do.
Usage data from published writing, books, news articles, professional documents, overwhelmingly favors 31st. 31th shows up almost exclusively in casual typos, never in edited or published material.
Comparison Table: Ordinal Number Endings
Here’s a broader breakdown so you can see the full pattern, not just the number 31.
| Last Digit | Suffix | Examples |
|---|---|---|
| 1 | -st | 1st, 21st, 31st |
| 2 | -nd | 2nd, 22nd, 32nd |
| 3 | -rd | 3rd, 23rd, 33rd |
| 4 to 9, 0 | -th | 4th, 15th, 20th, 30th |
| 11, 12, 13 | -th (exception) | 11th, 12th, 13th |
Once this table clicks, you can figure out almost any ordinal suffix without memorizing every single number individually. Just check the last digit, then check if it falls into the 11-13 exception zone.
Why Pronunciation and Spelling Usually Match
One thing worth mentioning: pronunciation and spelling typically align for ordinal numbers, and that’s actually a helpful shortcut. Say “thirty-first” slowly. You’ll hear that “st” sound naturally at the end, matching exactly what you write.
This connection between spoken form and written form works for most ordinal numbers, though not always perfectly. It’s still one of the fastest mental checks you can run when you’re typing quickly and want to double check yourself without pulling up a grammar guide.
Tips to Remember the Rule for Good
Here’s a simple trick that sticks. Group numbers by their last digit, then match them to the four suffixes: st, nd, rd, th. Memorize the 11-13 exception separately, since it breaks the pattern completely.
A few more tricks that help:
- Say the number out loud before writing it down
- Picture the exceptions (11th, 12th, 13th) as their own separate club
- Remember that 31 ends in 1, so it follows the same family as 1st and 21st
- When editing, search your document for “th” attached to numbers ending in 1, 2, or 3, since that catches most mistakes instantly
Grammar learners and non-native writers often find this rule tricky at first, but once it’s memorized, it becomes second nature within a few weeks of regular writing.
FAQs
Is it 31st or 31th? It’s 31st. There’s no situation where 31th is correct.
Why is it 31st instead of 31th? Because 31 ends in the digit 1, and numbers ending in 1 take the -st suffix, following the same rule as 1st and 21st.
Do all numbers ending in 1 use “st”? Almost all, except 11, which is an exception and always takes “th” instead.
Is the rule the same worldwide? Yes. Both British and American English use 31st, with no regional variation on this particular rule.
Can I just write “31” without the suffix? In some date formats, especially American style like “March 31,” dropping the suffix is common and accepted. But if you add a suffix, it must be -st.
Is 31th ever correct in any context? No. Not in casual writing, formal writing, or anywhere else. It’s simply not a recognized form.
Conclusion
So there you have it. 31st is the only correct form, and 31th doesn’t belong anywhere in standard, professional, or even casual English. The rule ties back to the last digit of the number, with a small set of exceptions for 11, 12, and 13 that you’ll want to memorize separately.
Whether you’re writing an email, scheduling an event, or just double checking a date before hitting send, remembering this one rule saves you from a small but noticeable mistake. Keep it simple: 31 ends in 1, so it takes -st, every single time.
