31st or 31th:Learn the Right Form

Should you write 31st or 31th? It’s a common question, especially when writing dates, invitations, emails, or official documents.

The correct form is 31st because it follows the standard rule for English ordinal numbers.

31th is not accepted in standard English and should be avoided.

In this guide, you’ll learn the correct spelling, why the rule works, how to pronounce it, and how to use 31st correctly in everyday writing.

Quick Answer

The correct form is 31st.

31th is incorrect and is not used in standard English.

Form	Correct?	Usage
31st	✅ Yes	Standard English
31th	❌ No	Incorrect

Quick takeaway: The ordinal form of 31 is always 31st because numbers ending in 1 usually take the -st suffix, except for 11th.

How to Spell and Pronounce 31st

Correct spelling

The correct ordinal form is:

31st

Written in words, it becomes:

Thirty-first

Pronunciation

IPA: /ˌθɜːr.ti ˈfɜːrst/

Simple pronunciation: THUR-tee FURST

Although 31st ends with st, you pronounce the full word as thirty-first, not “thirty-est.”

Where you’ll see it

You’ll commonly use 31st in:

Dates
Calendars
Invitations
Business emails
Legal documents
School assignments

Examples

Our meeting is on 31st August.
The report must be submitted by January 31st.
Her birthday falls on the 31st every year it appears in the calendar.

What Is the Ordinal Form of 31?

An ordinal number shows the position of something in a sequence.

The number 31 is a cardinal number because it tells “how many.”

The form 31st is an ordinal number because it tells “which one.”

Cardinal Number	Ordinal Number
31	31st
Thirty-one	Thirty-first

You use ordinal numbers whenever you talk about order, ranking, dates, or position.

Examples

She finished 31st in the marathon.
Today is the 31st of October.
He celebrated his 31st birthday with family.

Why Is It 31st and Not 31th?

Numbers ending in 1 usually take -st

Most ordinal numbers ending in 1 use the suffix -st.

Examples include:

1 → 1st
21 → 21st
31 → 31st
41 → 41st
101 → 101st

That’s why 31st is the correct form.

The exception to remember

The only common exception is the number 11.

Even though it ends in 1, the correct form is:

11th ✅
11st ❌

The same rule applies to:

12th
13th

These numbers always take -th.

Quick reference table

Number	Correct Form
1	1st
11	11th
21	21st
31	31st
41	41st
111	111th

Once you remember the 11, 12, and 13 exception, choosing the correct ordinal ending becomes much easier.

Ordinal Number Suffix Rules

English ordinal numbers follow a predictable pattern.

Number Ends In	Correct Suffix	Example
1	-st	1st, 21st, 31st
2	-nd	2nd, 22nd
3	-rd	3rd, 23rd
4–20	-th	4th, 15th, 20th
24–30	-th	24th, 30th
31	-st	31st

This rule is used consistently in modern English and is reflected in major dictionaries and editorial style guides.

How to Use 31st in Dates

British English

In British English, the day usually comes before the month.

Examples

31st January 2026
31st March
31st December

This format is common in the UK, Australia, and New Zealand.

American English

In American English, the month usually comes before the day.

Examples

January 31st, 2026
March 31st
December 31st

In many formal U.S. business and academic documents, you’ll also see January 31, 2026, where the ordinal suffix is omitted. Both styles are acceptable when they match the chosen style guide.

Real-life examples

Please RSVP by 31st May.
The subscription expires on January 31st.
Our office will remain closed until 31st December.
Applications received after 31st July will not be considered.

These examples show that 31st is the only correct ordinal form, regardless of whether you’re following British or American date conventions.

31st vs 31th: Real-Life Examples

The easiest way to remember the rule is to see it in context.

Correct examples

My passport expires on 31st December 2026.
The event is scheduled for January 31st.
She celebrated her 31st birthday with friends.
He finished 31st in the national competition.
Please submit your application before the 31st of this month.

Incorrect examples

❌ The meeting is on 31th March.
✅ The meeting is on 31st March.

❌ Today is 31th January.
✅ Today is 31st January.

❌ She celebrated her 31th birthday.
✅ She celebrated her 31st birthday.

If you’re writing about the number 31 as a position, rank, or date, 31st is always the correct choice.

Common Mistakes to Avoid

Writing 31th

This is the most common mistake.

❌ 31th
✅ 31st

Forgetting the exceptions

Many learners assume every number ending in 1 uses -st.

That works for 21st, 31st, and 41st, but 11th is an exception.

Number	Correct Form
11	11th
21	21st
31	31st

Mixing cardinal and ordinal numbers

A cardinal number tells how many.

An ordinal number tells position or order.

We invited 31 guests. ✅
The ceremony is on the 31st. ✅

Using the correct form makes your writing clearer and more professional.

A Simple Memory Trick

If the last digit is:

1 → -st
2 → -nd
3 → -rd
Everything else → -th

Then remember one exception:

11, 12, and 13 always end in -th.

This simple rule covers nearly every ordinal number you’ll write.

31st vs 31th Comparison Table

Feature	31st	31th
Correct spelling	✅ Yes	❌ No
Standard English	✅ Yes	❌ No
Ordinal number	✅ Yes	❌ No
Used in dates	✅ Yes	❌ No
Used in formal writing	✅ Yes	❌ No
Used in business documents	✅ Yes	❌ No
Used in academic writing	✅ Yes	❌ No
Pronunciation	Thirty-first	Not used

Frequently Asked Questions

Is 31th ever correct?

No. 31th is not a standard English spelling. The correct ordinal form is 31st.

Why is it 31st instead of 31th?

Because numbers ending in 1 usually take the -st suffix. Since 31 ends in 1, it becomes 31st.

How do you write 31st in words?

The correct written form is thirty-first.

How do you pronounce 31st?

It is pronounced thirty-first.

Is 31st an ordinal number?

Yes. It shows position or order rather than quantity.

Which is correct: 31st January or January 31st?

Both are correct.

31st January is common in British English.
January 31st is common in American English.

Can I use 31st in formal writing?

Yes. 31st is appropriate in emails, contracts, invitations, reports, and other formal documents when an ordinal date format is used.

Conclusion

The answer is straightforward: 31st is the correct ordinal form, while 31th is always incorrect.

English ordinal numbers follow clear suffix rules, and 31 takes -st because it ends in 1.

Whether you’re writing a date, describing a birthday, or mentioning a ranking, 31st is the form you should use.

Keep the 11th, 12th, and 13th exceptions in mind, stay consistent with your chosen date format, and you’ll avoid one of the most common grammar mistakes.