When you type out the number 31st or 31th as a ranking, a date, or an age, it is easy to pause and wonder whether it should end in “st” or “th.” Many writers second-guess themselves because other numbers, like 4th or 9th, always take “th,” so it feels natural to assume 31 should follow the same pattern.
The short answer is that 31st is the only correct spelling, and 31th is a common but avoidable mistake. This guide breaks down the rule behind ordinal numbers, walks through real examples, and clears up the confusion once and for all so you never have to second-guess it again.
Quick Answer: 31st Is Correct, 31th Is Not
Here is the direct comparison so you can settle the question in a few seconds.
| Form | Status | Reason |
| 31st | Correct | 31 is read as “thirty first,” and numbers ending in 1 take the -st suffix |
| 31th | Incorrect | There is no ordinal rule in English that assigns -th to numbers ending in 1 |
| Thirty-first | Correct (spelled out) | The full written form used in formal or narrative text |
The rule is simple once you see it laid out: ordinal suffixes are determined by the last digit of the number, not by habit or by how the word sounds when rushed.
Understanding Ordinal Numbers and Why 31 Follows the -st Pattern
Ordinal numbers describe position or order, such as first, second, and third, instead of quantity. Cardinal numbers like 31 tell you how many of something exist, while ordinal numbers like 31st tell you where something falls in a sequence.
Because 31 ends in the digit 1, it inherits the same suffix pattern as 1 (first) and 21 (twenty-first). English grammar does not restart the suffix rule at every new set of ten; instead, it repeats based on the final digit of the whole number.
- Numbers ending in 1 use -st, except for 11
- Numbers ending in 2 use -nd, except for 12
- Numbers ending in 3 use -rd, except for 13
- All other numbers use -th
This is why 31, 41, 51, and 61 all end in -st, following the same logic as the very first ordinal number.
Why People Mistakenly Write 31th Instead of 31st?
The mistake usually happens for one of three reasons: unfamiliarity with the -st and -nd exceptions, autocorrect habits carried over from casual typing, or confusion with the 11th, 12th, and 13th exception group. Because those three numbers break the normal pattern and always use -th, some writers assume every number is safer with -th by default.
Another factor is speed. When typing quickly on a phone or filling out a form, it is easy to default to the most common ordinal ending, -th, without registering that 31 needs the -st form. Recognizing this habit is the first step toward avoiding it consistently.
The 11th, 12th, and 13th Exception Explained
Every ordinal rule has one well known exception, and this is it. Even though 11, 12, and 13 end in 1, 2, and 3, they do not follow the standard -st, -nd, -rd pattern. Instead, they always take -th.
| Number | Expected Suffix by Pattern | Actual Correct Form |
| 11 | 11st | 11th |
| 12 | 12nd | 12th |
| 13 | 13rd | 13th |
| 111 | 111st | 111th |
| 112 | 112nd | 112th |
| 113 | 113rd | 113th |
This exception repeats at every hundred, so 111th, 211th, and 311th all behave the same way as 11th. Thirty-one, however, is not part of this exception group, which is exactly why it correctly reverts back to -st.
How to Write 31st in Words?
When spelling out the number in full, 31st becomes thirty-first, not thirty-oneth. The word “first” replaces “one” entirely in its ordinal form, the same way “one” becomes “first” and “twenty-one” becomes “twenty-first.”
- 31st in words: thirty-first
- 32nd in words: thirty-second
- 33rd in words: thirty-third
This pattern holds steady no matter how large the number grows, as long as the final digit stays the same.
Using 31st Correctly in Dates
Dates are one of the most common places people encounter this ordinal number, since December, January, March, May, July, August, and October all contain a 31st day.
- January 31st marks the final day of the first month of the year
- March 31st is often used as a fiscal quarter deadline
- December 31st is universally recognized as New Year’s Eve
In both American and British English, the numeral spelling stays the same. What differs between the two styles is only the order in which the day, month, and year appear, not the ordinal suffix itself.
31st in Everyday Contexts: Birthdays, Rankings, and Milestones
Beyond dates, 31st shows up constantly in daily language to describe age, position, and sequence.
- She just celebrated her 31st birthday with close friends
- He placed 31st out of more than two hundred runners in the marathon
- Today marks the 31st consecutive day of the savings challenge
- The company reported its 31st consecutive quarter of growth
Each of these examples uses 31st to indicate rank or sequence rather than a total count, which is the core function of every ordinal number.
American English Versus British English: Does the Spelling Change?
A frequent follow up question is whether regional English variants treat this number differently. They do not. Both American and British English use the identical spelling, 31st, and both consider 31th a spelling error.
The real difference between the two dialects is date formatting. American English typically writes month, day, year (March 31st, 2026), while British English usually writes day, month, year (31st March 2026). The ordinal number itself never changes.
Common Spelling Mistakes Related to 31st
Writers occasionally stumble on more than just the “th” versus “st” issue. Here are the most frequent variations to watch for:
- 31th, using -th where -st belongs
- 31rd, mistakenly borrowing the suffix used for numbers ending in 3
- 31nd, mistakenly borrowing the suffix used for numbers ending in 2
- 31 st, adding an unnecessary space between the number and suffix
Keeping the number and suffix joined without a space, and double checking the final digit, resolves nearly every version of this mistake.
A Simple Memory Trick to Never Get It Wrong Again
If you ever hesitate while typing, use this shortcut: ignore the tens digit and focus only on the very last number. Since 31 ends in 1, treat it exactly like the number 1, which becomes first. Extend that logic and you will land on 31st every time.
- 1 becomes 1st
- 21 becomes 21st
- 31 becomes 31st
- 41 becomes 41st
The only numbers that break this shortcut are 11, 12, and 13, so keep those three in the back of your mind as the sole exception to remember.
31st in Formal and Professional Writing
In business correspondence, academic papers, and legal documents, style guides generally recommend spelling out ordinal numbers when they appear at the start of a sentence, while numeral form (31st) is acceptable within the body of a sentence or in dates, invoices, and schedules.
- Formal prose: “The thirty-first amendment was proposed after lengthy debate.”
- Everyday or numeric writing: “The deadline falls on March 31st.”
Following this convention keeps formal documents polished while still allowing efficient, reader friendly numeral use elsewhere.
How Digital Tools Handle 31st Automatically?
Most modern word processors, spreadsheet programs, and grammar checking tools automatically apply the correct ordinal suffix when a date is entered or auto formatted. This built in behavior is one more confirmation that 31st, not 31th, is the industry standard spelling recognized across writing software, publishing platforms, and style guides alike.
Frequently Asked Questions
Is 31st or 31th correct?
31st is correct. 31th is not a recognized English spelling.
Why is 31st correct instead of 31th?
Because 31 ends in the digit 1, it follows the same -st pattern as first and twenty-first.
How do you spell 31st in words?
It is spelled thirty-first, not thirty-oneth.
Is 31th ever an accepted spelling?
No, it is considered incorrect in both American and British English.
What is the ordinal number for 31?
The ordinal number for 31 is 31st, or thirty-first when written out.
How should you write January 31st correctly?
Write it as January 31st, keeping the numeral and suffix joined without a space.
Does 31st change between American and British English?
No, only the date order changes; the ordinal spelling stays the same in both styles.
Conclusion
31st is the only correct way to write this ordinal number, and 31th should always be avoided. The rule is straightforward: numbers ending in 1 take -st, except for the 11th exception group, and 31 falls outside that exception.
Whether you are writing a birthday message, a deadline, or a formal report, remembering this one pattern will keep your writing accurate every time.
