Have you ever written “31th” and wondered if it looked incorrect? You’re not alone. Thousands of English learners and even native speakers occasionally make mistakes with ordinal numbers.
The confusion between 31st and 31th is one of the most common grammar errors when writing dates, rankings, and anniversaries.
In this guide, you’ll learn the correct form, understand the grammar rule behind it, and discover easy tips to avoid ordinal number mistakes forever.
Quick Answer
The correct form is 31st.
❌ 31th is incorrect and is not used in standard English.
This rule applies in both American English and British English.
Since the number 31 ends with the digit 1, it takes the ordinal suffix -st, just like:
- 1st
- 21st
- 41st
- 51st
- 101st
Correct Examples
✅ Her birthday is on March 31st.
✅ The contract expires on December 31st.
✅ He finished in 31st place in the race.
Incorrect Examples
❌ Her birthday is on March 31th.
❌ The contract expires on December 31th.
❌ He finished in 31th place.
What Does “31st” Mean?
The term 31st is an ordinal number. Ordinal numbers indicate the position of something in a sequence or order.
They answer questions such as:
- Which one?
- What position?
- In what order?
When you write 31st, you’re referring to the item, person, or date that comes after the 30th and before the 32nd.
Common Uses of 31st
- Dates: January 31st, March 31st
- Rankings: 31st position
- Editions: 31st edition of a publication
- Anniversaries: 31st wedding anniversary
- Birthdays: 31st birthday
Examples
✅ Today is January 31st, the last day of the month.
✅ This is the 31st annual conference.
✅ She celebrated her 31st birthday with friends.
Why Is It “31st” and Not “31th”?
English ordinal numbers follow a simple pattern based on the last digit of the number.
| Last Digit | Suffix | Example |
|---|---|---|
| 1 | -st | 1st, 21st, 31st |
| 2 | -nd | 2nd, 22nd, 32nd |
| 3 | -rd | 3rd, 23rd, 33rd |
| 4–9, 0 | -th | 4th, 10th, 20th |
Because 31 ends in 1, it must use -st.
The History of Ordinal Suffixes
The English language inherited many ordinal forms from Old English.
- -st comes from the word first
- -nd comes from second
- -rd comes from third
- -th became the standard ending for most other numbers
Over time, these forms evolved into the ordinal suffix system we use today.
Why Do People Write “31th”?
The mistake usually happens because -th is the most common ordinal ending in English.
People regularly see:
- 4th
- 5th
- 10th
- 20th
- 30th
As a result, many assume every number should use -th, leading to incorrect forms like 31th, 21th, and 41th.
British English vs American English
The good news is that both dialects agree completely on this rule.
Both Use:
✅ 31st
Neither Uses:
❌ 31th
The only difference is date formatting.
| Region | Date Format | Example |
|---|---|---|
| American English | Month + Day | January 31st, 2025 |
| British English | Day + Month | 31st January 2025 |
Regardless of format, the ordinal remains 31st.
31st vs Other Variations
| Form | Correct? | Usage |
|---|---|---|
| 31st | ✅ Correct | Dates, rankings, anniversaries |
| 31th | ❌ Incorrect | Never used |
| thirty-first | ✅ Correct | Formal writing |
| thirty-oneth | ❌ Incorrect | Not a valid word |
Which Form Should You Use?
For American English
Use:
- January 31st, 2025
- March 31st
For British English
Use:
- 31st January 2025
- 31st March 2025
For Academic Writing
Many style guides prefer spelling it out:
Thirty-first
However, 31st is also widely accepted.
For Social Media
Both of these are common:
✅ December 31st
✅ December 31
Common Mistakes with Ordinal Numbers
Many writers make similar errors:
❌ 21th → ✅ 21st
❌ 22th → ✅ 22nd
❌ 23th → ✅ 23rd
❌ 41th → ✅ 41st
❌ 51th → ✅ 51st
❌ 101th → ✅ 101st
Correction Table
| Wrong | Correct |
|---|---|
| March 31th | March 31st |
| 21th birthday | 21st birthday |
| 11st floor | 11th floor |
| thirty-oneth | thirty-first |
Examples of 31st in Everyday Writing
Emails
“Please submit your report by January 31st at 5 PM.”
Social Media
“Can’t believe it’s already December 31st! 🎉”
News Articles
“The summit is scheduled for March 31st, according to officials.”
School Assignments
“The science project is due on October 31st.”
Business Documents
“This offer remains valid until August 31st, 2025.”
Important Exception: 11th, 12th, and 13th
While ordinal suffixes usually depend on the last digit, there is one important exception.
Always use:
- 11th
- 12th
- 13th
Never write:
❌ 11st
❌ 12nd
❌ 13rd
These exceptions apply to all larger numbers ending in 11, 12, or 13 as well:
- 111th
- 212th
- 313th
Helpful Tips to Remember
Tip 1
Look at the last digit of the number.
Tip 2
Remember the special exception for 11, 12, and 13.
Tip 3
When unsure, write the number in words.
Example:
Thirty-first
Tip 4
Use grammar-checking tools to catch ordinal mistakes.
Frequently Asked Questions (FAQs)
Is “31th” ever correct?
No. 31th is never correct in standard English.
Why is it 31st and not 31th?
Because numbers ending in 1 use the suffix -st, except numbers ending in 11.
How do you write 31st in words?
Thirty-first
What is the ordinal form of 31?
31st or thirty-first
Does the same rule apply to 131?
Yes. The correct form is 131st.
Can I say “See you on the 31st”?
Absolutely. This is a common and correct expression.
Conclusion
The answer is straightforward: 31st is correct, and 31th is always incorrect.
Ordinal numbers in English follow specific suffix rules based on the last digit of a number. Since 31 ends in 1, it takes the -st suffix, making 31st the only correct form.
Whether you’re writing dates, rankings, anniversaries, emails, academic papers, or social media posts, always use 31st.
By remembering the simple ordinal number rules and the special exceptions for 11th, 12th, and 13th, you’ll never make this mistake again.
