31st is the correct form. 31th is incorrect because ordinal numbers ending in 1 (except 11) use the suffix -st, not -th.
Major dictionaries and English style guides follow this rule, so you should always write 31st when referring to dates, positions, or rankings.
Quick Answer Table
| Correct | Incorrect | Why |
|---|---|---|
| 31st | 31th | Numbers ending in 1 take -st (except 11). |
| 21st | 21th | Same ordinal number rule. |
| 41st | 41th | Last digit is 1, so use -st. |
| 51st | 51th | Follows the same pattern. |
Why People Make This Mistake
Unlike a simple spelling error, 31th is a grammar mistake involving ordinal numbers.
Many people learn that ordinal numbers usually end in -th, such as fourth, fifth, sixth, and tenth. Because -th appears so often, it’s easy to assume every number should follow the same pattern.
However, English uses different suffixes depending on the last digit of the number.
That’s why 31st is correct, even though 31th might look logical at first.
Another reason this mistake happens is that people focus on the whole number instead of its ending. In English, ordinal suffixes are determined mainly by the last digit, not by the number as a whole.
Editors commonly find this mistake in invitations, resumes, business emails, event posters, legal documents, and school assignments where dates are written frequently.
The Rule Behind It
Ordinal numbers tell us the position of something in a sequence.
Examples include:
- first
- second
- third
- fourth
When numbers are written as numerals, English adds a suffix.
The Basic Rule
| Last Digit | Suffix | Example |
|---|---|---|
| 1 | st | 21st, 31st, 101st |
| 2 | nd | 22nd, 32nd, 102nd |
| 3 | rd | 23rd, 33rd, 103rd |
| 4–9 | th | 24th, 25th, 39th |
| 0 | th | 10th, 20th, 30th |
Since 31 ends in 1, it becomes:
✅ 31st
not
❌ 31th
The Important Exception
There is one rule many people forget.
Numbers ending in 11, 12, and 13 always take -th, even though their last digit suggests otherwise.
Examples:
- 11th
- 12th
- 13th
But once you move past those exceptions, the normal rule returns.
Examples:
- 21st ✅
- 22nd ✅
- 23rd ✅
- 31st ✅
- 32nd ✅
- 33rd ✅
Remember this exception, and you’ll avoid most ordinal number mistakes.
How to Choose the Correct Ordinal Suffix
Whenever you’re unsure, follow these two simple steps.
Step 1
Look at the last two digits.
If the number ends in:
- 11
- 12
- 13
always use -th.
Step 2
If it doesn’t end in 11, 12, or 13, look at the last digit.
- 1 → st
- 2 → nd
- 3 → rd
- Everything else → th
Using this method:
31 ends in 1
It doesn’t end in 11
Therefore:
✅ 31st
Real-Life Examples
Dates
Our appointment is scheduled for 31st August.
Invitations
Please RSVP before 31st December.
Business Emails
The report must be submitted by 31st March.
Legal Documents
The agreement becomes effective on the 31st day of January.
Workplace Communication
Payroll will be processed on the 31st of this month.
News Writing
Applications close on 31st July, according to the official announcement.
Incorrect vs Correct
❌ The event will be held on 31th October.
✅ The event will be held on 31st October.
❌ Submit your application before 31th May.
✅ Submit your application before 31st May.
Common Places This Mistake Appears
Wedding Invitations
Dates on invitations are often typed quickly without checking the ordinal suffix.
Example: The ceremony will take place on 31th October.
Business Emails
Professionals frequently mention deadlines using calendar dates.
Example: Please submit the invoice by 31th March.
School Assignments
Students sometimes apply -th to every number without remembering the ordinal rule.
Example: The project is due on 31th January.
Legal Documents
Contracts and agreements require accurate date formatting.
Example: This agreement expires on 31th December.
Event Posters
Promotional materials often include dates that should be carefully proofread.
Example: Registration closes on 31th July.
Online Forms
Users may accidentally type the wrong suffix when entering dates.
Example: My appointment is on 31th August.
Social Media Posts
Informal writing can lead to small grammar mistakes.
Example: Can’t wait for 31th December!
Blog Posts
Content creators occasionally overlook ordinal number rules.
Example: The giveaway ends on 31th May.
Workplace Chats
Quick messages often contain date-related errors.
Example: Let’s finish everything before 31th January.
Tip: Every example above should use 31st, not 31th.
Memory Trick
Here’s an easy way to remember it:
If the number ends in 1, think “first” and use “st.”
So:
- 1st
- 21st
- 31st
- 41st
All follow the same pattern.
Just remember one exception:
11th, not 11st.
A second trick is to read the number aloud.
You naturally say:
Thirty-first
You never say:
Thirty-th.
If it sounds wrong when spoken, it’s probably wrong when written.
Mini Quiz
1. Which is correct?
A. 31th
B. 31st
2. Fill in the blank.
The deadline is _ July.
3. Which sentence is correct?
A. Our meeting is on 31th March.
B. Our meeting is on 31st March.
4. True or False
Numbers ending in 1 always use -st.
Answer Key
- ✅ B
- ✅ 31st
- ✅ B
- ✅ False (11th is the exception.)
FAQs
Is 31th ever correct?
No. 31th is never correct in standard English. The correct ordinal form is 31st.
Which is correct: 31st or 31th?
31st is correct because numbers ending in 1 use -st, unless the number ends in 11.
Why is it 31st instead of 31th?
English ordinal numbers follow the last digit in most cases. Since 31 ends in 1, it takes -st. The only exception is numbers ending in 11.
How do you write the date 31 correctly?
Write it as 31st when using numerals.
Example: 31st December 2026
Is 31st correct in both British and American English?
Yes. Both British and American English use 31st. The difference is usually in the order of the date, not the ordinal suffix.
Can I write just 31 instead of 31st?
Yes. In many styles, especially American English, dates are commonly written as July 31 instead of 31st July. Follow your preferred style guide.
Do I always need to write st after 31?
No. If you’re simply writing the number, 31 is enough. Use 31st only when an ordinal number is required.
Why do ordinal numbers use different endings?
English ordinal numbers developed over time from words like first, second, and third, which is why they don’t all use the same suffix.
What is the rule for st, nd, rd, and th?
Use -st for numbers ending in 1, -nd for 2, -rd for 3, and -th for most other numbers, except 11, 12, and 13, which always use -th.
Is 31th accepted in British English?
No. 31th is incorrect in every standard variety of English.
Does Grammarly flag 31th?
Yes. Grammarly and most grammar-checking tools identify 31th as an incorrect ordinal and recommend 31st.
Should I write 31st in formal writing?
Yes. 31st is appropriate in invitations, contracts, academic writing, emails, and other formal documents whenever an ordinal number is needed.
How do you pronounce 31st?
It is pronounced thirty-first.
What comes after 30th?
The next ordinal number is 31st, followed by 32nd and 33rd.
Is 31st used for rankings as well as dates?
Yes. Ordinal numbers indicate position as well as dates.
Example: She finished in 31st place.
Related Mistakes You Might Also See
If this mistake happened because of ordinal number suffixes, you may also find these guides helpful:
- 1st or 1th
- 2nd or 2th
- 3rd or 3th
- 21st or 21th
- 22nd or 22th
- 23rd or 23th
- 32nd or 32th
- 33rd or 33th
- 41st or 41th
- 42nd or 42th
- 43rd or 43th
- 101st or 101th
All of these follow the same ordinal number rules explained in this guide.
Conclusion
The correct form is 31st. Writing 31th is a common mistake caused by applying the -th ending to every number instead of following English ordinal rules.
A simple way to remember the rule is to check the last digit. If the number ends in 1, use -st—unless it ends in 11, which is one of the special exceptions.
Once you learn this pattern, you’ll also know how to write numbers like 21st, 41st, and 101st correctly.
Related Blogs:
