Quick Answer: “31st” is correct. “31th” is always wrong.
The rule comes down to pronunciation. You say “thirty-first,” not “thirty-th,” so the written form takes the st suffix — the same suffix used for 1st, 21st, and every other number ending in 1 (except 11). If you remember nothing else from this guide, remember this: 31st, never 31th.
| ✅ Correct | ❌ Incorrect |
|---|---|
| 31st | 31th |
| December 31st | December 31th |
| 31st birthday | 31th birthday |
The Ordinal Number Rule (And Why 31 Takes “st”)
Ordinal numbers describe position — first, second, third — rather than quantity. In English, four suffixes cover almost every number:
| Last Digit of Number | Suffix | Examples |
|---|---|---|
| 1 | -st | 1st, 21st, 31st, 41st |
| 2 | -nd | 2nd, 22nd, 32nd |
| 3 | -rd | 3rd, 23rd, 33rd |
| 4–9 and 0 | -th | 4th, 15th, 20th, 30th |
| Exception: 11, 12, 13 | -th | 11th, 12th, 13th |
Why 31 breaks the “th” pattern: Say the number out loud — thirty-first. The word “first” is baked into the pronunciation, so the abbreviation has to end in -st to match. This is the same reason 21 is “twenty-first” → 21st, and 41 is “forty-first” → 41st. The pattern isn’t arbitrary; it’s phonetic.
💡 Quick test: If you’re ever unsure which suffix to use, say the number in full words. Whatever the last word sounds like (first, second, third, fourth…) tells you the correct suffix.
31st in Real Sentences: Examples That Make It Stick
Seeing the rule in context makes it easier to remember than seeing it in isolation.
✅ Correct usage:
- My grandmother is celebrating her 31st birthday for the tenth year in a row (she jokes about it every time).
- The runner finished in 31st place out of two hundred competitors.
- Rent is due on the 31st of every month with 31 days.
- The company’s fiscal year closes on December 31st.
❌ What it should never look like:
- ~~My grandmother is celebrating her 31th birthday.~~
- ~~The runner finished in 31th place.~~
- ~~Rent is due on the 31th.~~
Notice that the correct examples read naturally when spoken aloud — “thirty-first birthday,” “thirty-first place.” The incorrect versions don’t; nobody says “thirty-th.”
Where the Rule Applies: Dates, Lists, Formal Writing
A common misconception is that the rule shifts depending on context — that maybe dates work differently than rankings, or that British English handles it differently than American English. It doesn’t. 31st is correct everywhere, in every context, with no exceptions.
| Context | Correct Form | Example |
|---|---|---|
| Dates | 31st | “The event is on October 31st.” |
| Birthdays / anniversaries | 31st | “It’s her 31st anniversary.” |
| Rankings / lists | 31st | “He placed 31st in the marathon.” |
| Formal / business writing | 31st | “Submit the report by January 31st.” |
| Emails | 31st | “Let’s meet on the 31st.” |
| Social media / informal writing | 31st | “Countdown to the 31st!” |
This also resolves month-specific searches people commonly run into:
- “December 31st or 31th” → December 31st
- “March 31st or 31th” → March 31st
- “January 31st or 31th” → January 31st
- “October 31st or 31th” → October 31st
The month never changes the ordinal ending — only the last digit of the number matters.
Common Mistakes People Make (And How to Catch Them)
- Mistake 1: Assuming every ordinal ends in “th.” Wrong: 21th, 31th, 41th Right: 21st, 31st, 41st
- Mistake 2: Forgetting the 11–13 exception. Numbers ending in 1, 2, or 3 usually take st/nd/rd — except when the number itself is 11, 12, or 13. Those always take “th” (11th, 12th, 13th), regardless of what comes before them (111th, 213th).
- Mistake 3: Mixing up “rd” and “st.” Wrong: 31rd Right: 31st (rd is reserved for numbers ending in 3, like 3rd and 23rd)
- Mistake 4: Inserting a space or extra letters. Wrong: 31 st, 31sth Right: 31st (no space, no extra characters)
Quick self-check: Read the number aloud. If it ends in “-first,” write st. If your written suffix doesn’t match how you’d say it, you’ve made a mistake.
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British vs American English: Any Difference?
None. This is one of the few areas of English spelling where there’s no regional split.
| Number | British English | American English |
|---|---|---|
| 31st | Correct | Correct |
| 31th | Incorrect | Incorrect |
Whether you’re writing for a UK audience, a US audience, or anywhere else, the rule holds: 31st is universal.
Frequently Asked Questions
Is 31th ever correct?
No. In standard English, under no circumstance is “31th” a correct spelling.
Why is it 31st and not 31th?
Because the number is pronounced “thirty-first,” and the written suffix follows the spoken word — “first” becomes “-st.”
Can it be 31rd?
No. “-rd” is reserved for numbers ending in 3, such as 3rd and 23rd. Thirty-one ends in 1, so it takes “-st.”
Should I write “31 st” with a space, or “31th”?
Neither. The only correct format is “31st” — no space, no “th.”
How do you write 31 in ordinal words?
“Thirty-first.” The numeral form is 31st.
Do all numbers ending in 1 use “st”?
Yes, with one exception: 11 always takes “th” (11th), even though it ends in 1.
Is the rule different for dates versus rankings?
No. Context doesn’t change the suffix — only the last digit(s) of the number do.
Why do so many people search “31th” instead of “31st”?
Most likely because they’ve generalized the “-th” ending (used for 4th–9th and 0) to all numbers, without accounting for the 1, 2, and 3 exceptions.
Final Takeaway
Always use 31st. Never use 31th. The rule is simple, consistent, and doesn’t change based on month, context, region, or writing style — spoken “thirty-first” gives you the written “31st.” Once this pattern clicks, you’ll be able to apply the same logic to every other ordinal number in English, from 1st to 101st and beyond.
