21th or 21st is a common spelling question, but the correct form is 21st. You should write “21st November,” “21st birthday,” and “21st century,” not “21th November,” “21th birthday,” or “21th century.” The difference comes from ordinal numbers, which show the position of something in a sequence. Once you understand the rules for 1st, 2nd, 3rd, and th, you can also avoid mistakes such as 22th, 23th, 31th, and 11st.
Whether you’re writing a birthday invitation, a calendar entry, a school assignment, or a professional document, the right ordinal ending helps your writing look clear and natural. This guide explains the difference between 21th and 21st, shows how to use the correct form with different months, and covers the rules behind other common ordinal number spelling mistakes.
Is It 21th or 21st?
The correct spelling is 21st, not 21th. The number 21 takes the ordinal ending -st because its final digit is 1, following the pattern of 1st, 21st, and 31st. You should use 21st when referring to a date, birthday, century, or position in a sequence. For example, “My 21st birthday is next week” is correct.
The Correct Spelling Is 21st
21st is the standard English ordinal form of the number 21. It represents the position immediately after twentieth and immediately before twenty-second.
Here are some examples of the correct usage:
- Today is the 21st of October.
- She celebrated her 21st birthday with her family.
- We live in the 21st century.
- He finished in 21st place in the competition.
- The meeting will take place on the 21st day of the month.
In each example, 21st identifies a position or order rather than simply counting an amount.
You can also write the number in words as twenty-first. Both forms represent the same ordinal number, but numerals are common in dates, lists, schedules, and informal writing.
Why 21th Is Incorrect
The form 21th does not follow standard English ordinal-number rules. Although many numbers use the ending -th, such as 10th, 20th, and 24th, numbers ending in 1 usually take -st.
Compare these forms:
| Incorrect spelling | Correct spelling |
| 1th | 1st |
| 21th | 21st |
| 31th | 31st |
| 41th | 41st |
| 51th | 51st |
The important detail is that the ending depends on the number’s final digit, except for numbers ending in 11, 12, or 13.
For example, 21 ends in 1, so it becomes 21st. However, 11 ends in 11, so the correct form is 11th. This exception explains why the rule works for many numbers but not every number that contains a final 1.
What Does 21st Mean?
21st is an ordinal number that means “twenty-first.” It identifies the position of a person, object, event, or period in a sequence.
For example, if 21 runners finish a race and one runner crosses the finish line in the twenty-first position, that runner finishes 21st. The number does not tell you how many runners participated; it tells you the runner’s position.
Here is the difference between cardinal and ordinal numbers:
- Cardinal number: 21 students attended the class.
- Ordinal number: The 21st student entered the classroom.
- Cardinal number: She bought 21 books.
- Ordinal number: This is her 21st book.
Understanding this distinction makes it easier to decide when to write 21 and when to write 21st.
Why Do We Write 21st Instead of 21th?
We write 21st instead of 21th because English ordinal numbers use different endings to express sequence. Numbers ending in 1 generally use -st, numbers ending in 2 use -nd, and numbers ending in 3 use -rd. Most other numbers use -th. However, 11, 12, and 13 always take -th, regardless of their final digits.
Understanding Ordinal Numbers
An ordinal number tells you where something belongs in a sequence. You use ordinal numbers when describing dates, rankings, floors, chapters, birthdays, and centuries.
Common examples include:
- 1st: first
- 2nd: second
- 3rd: third
- 4th: fourth
- 10th: tenth
- 21st: twenty-first
- 22nd: twenty-second
- 23rd: twenty-third
- 24th: twenty-fourth
Ordinal numbers appear in everyday writing more often than you might realize. A student may read the third chapter of a book, an athlete may finish in second place, and a family may celebrate someone’s twenty-first birthday.
The suffix changes to match the ordinal form. You cannot simply attach -th to every number, which is why 21th is incorrect.
The Difference Between Cardinal and Ordinal Numbers
Cardinal and ordinal numbers serve different purposes.
Cardinal numbers answer the question “How many?” Ordinal numbers answer the question “Which one in order?”
| Cardinal number | Ordinal number | Example |
| 1 | 1st | The 1st runner crossed the line. |
| 2 | 2nd | She finished in 2nd place. |
| 3 | 3rd | He opened the 3rd box. |
| 11 | 11th | The 11th player joined the team. |
| 21 | 21st | The 21st guest arrived late. |
| 31 | 31st | The 31st entry won the prize. |
Use cardinal numbers when counting quantities. Choose ordinal numbers when the order matters.
For example, “I have 21 books” describes a quantity, while “I read the 21st chapter” identifies a position in a book.
Why 21st Follows the Same Pattern as 1st
The number 21 follows the same ordinal pattern as 1 because its final digit is 1, and it does not fall into the special 11–13 group.
The same pattern appears in:
- 1st
- 21st
- 31st
- 41st
- 51st
- 61st
- 71st
- 81st
- 91st
However, you must check the final two digits before applying the rule. The numbers 111, 211, and 311 take the ending -th because they end in 11.
For example:
- 21st is correct.
- 111th is correct.
- 121st is correct.
- 211th is correct.
- 221st is correct.
This distinction prevents a common error: assuming that every number ending in the digit 1 automatically takes -st.
21th or 21st in Dates
The correct form is 21st when writing an ordinal date, such as “21st November” or “the 21st of September.” You can also write a date without an ordinal suffix, as in “November 21” or “September 21.” Both styles are standard, but you should choose a format that suits the sentence and follow it consistently.
21th or 21st November
The correct ordinal form is 21st November, not 21th November.
In British English, writers commonly place the day before the month. In American English, writers usually place the month before the day.
Examples:
- The conference begins on 21st November.
- Her appointment is scheduled for November 21.
- We plan to travel on November 21st.
- The application deadline is the 21st of November.
All these examples communicate the same calendar date. The main difference lies in formatting and regional style.
In formal writing, a style guide may prefer “21 November” or “November 21” without an ordinal suffix. You should not assume that every correct date must contain -st.
Remember: If you use an ordinal suffix, write 21st, not 21th.
21th or 21st September
The correct ordinal form is 21st September.
Use this form when the day comes before the month, or write “September 21st” when the month comes first and you want to include the suffix.
Examples:
- The school term begins on 21st September.
- Our family reunion is scheduled for September 21st.
- The notice was issued on 21 September.
- She submitted the application on September 21.
Notice that the third and fourth examples omit the ordinal suffix. That is a normal date style, not a spelling error.
When you write a date for an official notice, business letter, or academic document, follow the format used throughout the document.
21th or 21st December
The correct ordinal form is 21st December.
Here are a few natural examples:
- The winter concert will take place on 21st December.
- The store will close early on December 21st.
- We booked our tickets for 21 December.
- The final payment is due on December 21.
December dates often appear in holiday schedules, travel bookings, school calendars, and event announcements. In each case, use 21st if you choose an ordinal format.
A date such as “21th December” is incorrect even if the month and year are otherwise written correctly.
21th or 21st August
Write 21st August, not 21th August.
Examples include:
- The course starts on 21st August.
- The ceremony will be held on August 21st.
- The company announced the change on 21 August.
- Their holiday begins on August 21.
If you’re preparing an invitation, the ordinal form can sound natural: “Join us on the 21st of August.” For a concise schedule or business notice, “August 21” may be a better fit.
Both choices are acceptable. The key is to use the correct ordinal form whenever you include a suffix.
21th or 21st June
The correct form is 21st June.
Examples:
- The summer program starts on 21st June.
- Her examination is scheduled for June 21st.
- The report must be submitted by 21 June.
- We will meet on June 21.
The month does not change the ordinal-number rule. Whether the date falls in June, August, September, November, or December, 21 takes the ending -st.
Should You Write November 21st or November 21?
Both November 21st and November 21 are acceptable date formats. The choice depends on the style you are following, the type of document, and your regional writing conventions.
Use these examples as a guide:
| Date format | Typical use |
| November 21 | American business writing, news, and many formal documents |
| November 21st | Informal writing, invitations, and conversational contexts |
| 21 November | British and international formal writing |
| 21st November | British-style informal writing and invitations |
| The 21st of November | Natural in sentences and spoken-style writing |
For example, a business email might say, “The report is due November 21.” A birthday invitation might read, “Join us on November 21st to celebrate.”
Both sentences are correct. However, you should avoid mixing styles unnecessarily in the same document.
For instance, if a schedule lists “November 5,” “November 12,” and “November 21,” there is no need to add suffixes to only the final date.
Is It 21th or 21st Birthday?
The correct phrase is 21st birthday, not 21th birthday. The ordinal number identifies the birthday as the twenty-first anniversary of a person’s birth. You can use “21st birthday” in invitations, cards, captions, and messages. You can also spell it out as “twenty-first birthday” when the context or writing style calls for words rather than numerals.
The Correct Phrase Is 21st Birthday
A birthday marks the anniversary of someone’s birth. The ordinal number identifies which anniversary the person is celebrating.
Examples:
- Today is my 21st birthday.
- We’re planning a surprise party for her 21st birthday.
- He received a watch for his 21st birthday.
- They invited all their friends to the 21st birthday celebration.
The suffix belongs to the number, not the word “birthday.” You therefore write 21st birthday as two words, with no additional ending attached to birthday.
Avoid these incorrect forms:
- 21th birthday
- 21th Birthday Party
- 21th bday
Use 21st birthday, 21st birthday party, or 21st birthday celebration instead.
Examples of 21st Birthday Wishes and Invitations
The phrase appears in many kinds of personal writing. Here are original examples you can adapt.
Birthday wishes
- Happy 21st birthday! May the year ahead bring you exciting opportunities and wonderful memories.
- Wishing you a fantastic 21st birthday filled with laughter, love, and new adventures.
- Cheers to your 21st birthday and everything you’re looking forward to in the years ahead.
Invitation wording
- Join us to celebrate Emma’s 21st birthday on Saturday, November 21st.
- You’re invited to a special 21st birthday dinner with family and friends.
- Save the date for a 21st birthday celebration on September 21.
Social media captions
- Twenty-one looks good on you!
- Celebrating 21 years of memories and new beginnings.
- Officially celebrating my 21st birthday today.
These examples demonstrate how the ordinal form fits naturally into complete sentences and short messages.
Should You Write 21st Birthday or Twenty-First Birthday?
Both 21st birthday and twenty-first birthday are correct. The numerical form is compact and common in invitations, headlines, captions, and everyday writing. The written form may suit formal prose, literary writing, or a sentence in which spelling out numbers improves readability.
Compare these examples:
- We’re celebrating her 21st birthday this weekend.
- We’re celebrating her twenty-first birthday this weekend.
The meaning is identical.
When choosing between the two, consider your audience and the format of the text. A printed invitation may use “21st Birthday Celebration” as a heading, while a more formal narrative may spell out “twenty-first birthday.”
Do not write “twenty-oneth.” The written ordinal is twenty-first, just as the numerical form is 21st.
21th or 21st Century?
The correct expression is 21st century, not 21th century. The phrase refers to the period from January 1, 2001, through December 31, 2100, under the standard Gregorian calendar system. Use “21st century” when discussing modern history, technology, culture, or society. In compound adjectives, write “21st-century technology.”
Why the Correct Term Is 21st Century
A century is a period of 100 years. The ordinal number identifies which hundred-year period you’re discussing.
The 21st century began on January 1, 2001, and will end on December 31, 2100. It did not begin in 2000 because the traditional calendar system has no year zero.
Here is the sequence:
| Century | Years covered |
| 19th century | 1801–1900 |
| 20th century | 1901–2000 |
| 21st century | 2001–2100 |
| 22nd century | 2101–2200 |
This timeline explains why “21st century” is correct. The suffix follows the ordinal-number rule, while the dates show the historical period.
Examples:
- Artificial intelligence is changing many aspects of 21st-century life.
- Education has evolved considerably in the 21st century.
- Historians study the early years of the 21st century to understand recent social changes.
Should You Capitalize 21st Century?
In ordinary prose, write 21st century in lowercase unless it appears at the beginning of a sentence or forms part of a proper name or title that uses capitalization.
Examples:
- Technology has transformed life in the 21st century.
- Many schools are adapting to 21st-century learning.
- The 21st century has brought major changes in communication.
The compound adjective “21st-century” usually takes a hyphen when it directly modifies a noun.
Compare:
- The world is changing in the 21st century.
- The company develops 21st-century technology.
The hyphen helps connect the words when they work together as an adjective. You don’t need it when the phrase simply identifies the century.
How to Use 21st Century in a Sentence
Use “21st century” when discussing events, ideas, or developments associated with the current century.
Examples:
- Healthcare in the 21st century increasingly relies on digital records and advanced diagnostic tools.
- Businesses must adapt to 21st-century customer expectations.
- Many people use online platforms to learn new skills in the 21st century.
- The museum explores how art changed during the early 21st century.
For comparison, “21th century” is incorrect in every one of these contexts. The correct form remains 21st century, whether the phrase appears in a school essay, a research paper, a website, or a casual conversation.
21st, 22nd, 23rd, or 24th? Ordinal Number Rules
English ordinal numbers generally use -st for numbers ending in 1, -nd for numbers ending in 2, -rd for numbers ending in 3, and -th for most other numbers. The main exception applies to numbers ending in 11, 12, or 13. These always use -th, including 11th, 112th, and 213th.
When to Use -st
Use -st for ordinal numbers that end in 1, except those ending in 11.
Examples:
- 1st
- 21st
- 31st
- 41st
- 51st
- 101st
- 121st
- 201st
In words, these are first, twenty-first, thirty-first, forty-first, and so on.
The number 121 takes -st because its final two digits are 21. The same applies to 201, which ends in 01 rather than 11.
Practical tip: Check the final two digits before adding a suffix. This quick check prevents you from incorrectly writing 111st instead of 111th.
When to Use -nd
Use -nd for numbers ending in 2, except those ending in 12.
Examples:
- 2nd
- 22nd
- 32nd
- 42nd
- 52nd
- 102nd
- 122nd
- 202nd
Their written forms include second, twenty-second, thirty-second, and forty-second.
For example, “The team finished 22nd in the tournament” is correct. “The team finished 22th” is not.
Remember that 112 takes the ending -th because its last two digits are 12.
When to Use -rd
Use -rd for numbers ending in 3, except those ending in 13.
Examples:
- 3rd
- 23rd
- 33rd
- 43rd
- 53rd
- 103rd
- 123rd
- 203rd
These forms correspond to third, twenty-third, thirty-third, and so on.
For example, “She finished in 23rd place” is correct, while “She finished in 23th place” is incorrect.
The same exception applies to 113, 213, and 313. Each ends in 13, so each takes -th.
When to Use -th
Use -th for most ordinal numbers that do not qualify for the -st, -nd, or -rd endings.
Examples include:
- 4th
- 5th
- 6th
- 10th
- 20th
- 24th
- 30th
- 40th
- 100th
- 110th
This ending also applies to the special numbers 11, 12, and 13.
For example:
- The 4th chapter contains the answer.
- The meeting is on the 10th of May.
- She celebrated her 20th birthday last year.
- The team finished in 24th place.
A frequent mistake is to apply the ending based only on the last digit. Always check for the 11, 12, and 13 exception first.
The Exception for Numbers Ending in 11, 12, and 13
Numbers ending in 11, 12, or 13 take -th, regardless of the final digit.
| Number | Correct ordinal | Reason |
| 11 | 11th | Ends in 11 |
| 12 | 12th | Ends in 12 |
| 13 | 13th | Ends in 13 |
| 111 | 111th | Ends in 11 |
| 112 | 112th | Ends in 12 |
| 113 | 113th | Ends in 13 |
| 211 | 211th | Ends in 11 |
| 212 | 212th | Ends in 12 |
| 213 | 213th | Ends in 13 |
Compare these examples:
- 21st, but 11th
- 22nd, but 12th
- 23rd, but 13th
- 121st, but 111th
- 122nd, but 112th
- 123rd, but 113th
This exception is one of the most important details in English ordinal spelling. It explains why 111st, 112nd, and 113rd are incorrect.
22th or 22nd, 31th or 31st, and Other Common Mistakes
The correct forms are 22nd, 31st, 23rd, and 11th. These mistakes happen when writers attach -th to every number or follow the last digit without checking the 11–13 exception. A simple ordinal-number table makes the correct forms easier to recognize and remember.
22th or 22nd: Which Is Correct?
22nd is correct because 22 ends in 2 and does not fall into the 12 exception.
Examples:
- She finished in 22nd place.
- The event is scheduled for the 22nd of November.
- He celebrated his 22nd birthday last weekend.
The form 22th is incorrect in standard English. When writing the number in words, use “twenty-second.”
31th or 31st: Which Is Correct?
31st is correct because 31 ends in 1 and does not end in 11.
Examples:
- The deadline is the 31st of August.
- She finished in 31st place.
- The calendar shows the 31st day of the month.
However, not every month has 31 days. February has 28 days in a common year and 29 in a leap year, while April, June, September, and November have 30 days.
This is a useful distinction: 31st is the correct ordinal spelling, but whether a 31st date exists depends on the month.
23rd or 23th: Which Is Correct?
23rd is correct because 23 ends in 3 and does not fall into the 13 exception.
Examples:
- The ceremony begins on the 23rd of September.
- He finished in 23rd place.
- Their anniversary falls on the 23rd day of the month.
The spelling “23th” is incorrect. When written out, use “twenty-third.”
Compare 23rd with 13th. Although both numbers end in 3, 13 takes -th because it belongs to the special 11–13 group.
11th or 11st: Which Is Correct?
11th is correct. The numbers 11, 12, and 13 are exceptions to the normal ordinal-ending pattern, so they use -th.
Examples:
- The appointment is on the 11th of December.
- She finished in 11th place.
- His birthday falls on the 11th.
Do not write 11st. Although 1st is correct, the same ending does not apply to 11.
The rule also applies to larger numbers ending in 11, such as 111th and 211th.
Quick Reference Table for Common Ordinal Numbers
Use this table when you need to check an ordinal number quickly.
| Incorrect form | Correct form | Written form |
| 1th | 1st | First |
| 2th | 2nd | Second |
| 3th | 3rd | Third |
| 11st | 11th | Eleventh |
| 12nd | 12th | Twelfth |
| 13rd | 13th | Thirteenth |
| 21th | 21st | Twenty-first |
| 22th | 22nd | Twenty-second |
| 23th | 23rd | Twenty-third |
| 24rd | 24th | Twenty-fourth |
| 31th | 31st | Thirty-first |
| 32th | 32nd | Thirty-second |
| 33th | 33rd | Thirty-third |
| 111st | 111th | One hundred eleventh |
| 112nd | 112th | One hundred twelfth |
| 113rd | 113th | One hundred thirteenth |
The table highlights the two main rules: use the ending that matches the ordinal form, and make an exception for numbers ending in 11, 12, or 13.
How to Write Ordinal Numbers Correctly in Everyday Writing
Correct ordinal numbers make dates, invitations, rankings, and formal documents easier to read. To avoid errors, identify whether you need a cardinal or ordinal number, check the final two digits, and apply the appropriate suffix. Then review the surrounding sentence to make sure the date or sequence makes sense.
Ordinal Numbers in Dates and Invitations
Dates are among the most common places where people use ordinal numbers.
Examples:
- Your appointment is on the 21st of November.
- The wedding will take place on June 23rd.
- Please submit your form by the 31st of August.
- The invitation is for a 21st birthday celebration.
When writing invitations, you can choose a more conversational style, such as “Join us on the 21st of November,” or a shorter format, such as “Saturday, November 21.”
If you include a suffix, check it carefully. A beautifully designed invitation can still contain an obvious spelling error if it says “21th birthday.”
Also remember that date styles vary by region. British English commonly uses day-month order, while American English commonly uses month-day order. Neither convention changes the ordinal spelling.
Ordinal Numbers in Academic and Professional Writing
Ordinal numbers also appear in reports, essays, research documents, instructions, and presentations.
Examples:
- The results appear in the 3rd section of the report.
- The committee will meet for the 21st time this year.
- The company opened its 100th location in June.
- Researchers examined developments in the 20th and 21st centuries.
For formal writing, check your organization’s or publication’s style guide. Some guides prefer numerals with suffixes, while others favor words in particular contexts.
For example, a report may use “the third quarter” in prose but “Q3” in a financial table. The best choice depends on the document’s conventions.
Consistency matters. If you use “21st century” throughout a report, avoid switching randomly to “twenty-first century” unless the change serves a clear stylistic purpose.
When to Spell Out Numbers Instead of Using Numerals
Both numerals and written ordinal numbers are correct, but each format has its place.
| Context | Common choice | Example |
| Calendar entry | Numeral | November 21 |
| Birthday invitation | Numeral with suffix | 21st birthday |
| Narrative prose | Words or numerals | Her twenty-first birthday |
| Ranking list | Numeral with suffix | 21st place |
| Historical discussion | Numeral with suffix | 21st century |
| Formal date | Numeral without suffix in many styles | 21 November |
These are common choices, not universal rules. A particular publication may have its own number-formatting requirements.
The most important point is to distinguish the correct spelling from the preferred style. “21st” is the correct ordinal form, while “November 21” is also a correct date format without an ordinal suffix.
Common Punctuation and Formatting Mistakes to Avoid
Ordinal numbers are simple once you know the rules, but a few formatting errors can still make your writing look inconsistent.
Avoid incorrect suffixes. Write 21st, not 21th, and 22nd, not 22th.
Do not add a suffix to every date automatically. “November 21” is correct without -st.
Use hyphens in compound adjectives when needed. Write “21st-century technology” but “technology in the 21st century.”
Keep number styles consistent. If your document uses “March 21,” “April 15,” and “May 30,” you generally do not need to add ordinal suffixes to one date.
Check the special 11–13 rule. Write 111th, 112th, and 113th, not 111st, 112nd, or 113rd.
A quick proofreading check can catch these issues before you send an email, publish an article, or print an invitation.
Frequently Asked Questions
Is 21th Ever Correct?
No. 21st is the standard English ordinal form of 21. Use it for dates, birthdays, rankings, and centuries. The form 21th is a common spelling mistake caused by applying the -th ending too broadly.
Is It 21st or 21th November?
21st November is correct when you use an ordinal suffix. You can also write “21 November” without the suffix. In American English, “November 21” is common, while “November 21st” is also used in informal writing.
Is It 21th or 21st Birthday?
The correct phrase is 21st birthday. For example, “We’re celebrating her 21st birthday this Saturday.” You can also spell it out as “twenty-first birthday.” Avoid writing “21th birthday.”
Is It 21th or 21st Century?
21st century is correct. The 21st century covers the years 2001 through 2100. When the phrase works as an adjective, use a hyphen: “21st-century technology.” The form 21th century is incorrect.
Why Is 11th Correct Instead of 11st?
11th is correct because numbers ending in 11, 12, or 13 take the -th suffix. This exception applies even when the number has more than two digits. For example, 111th and 211th are correct, not 111st or 211st.
What Is the Difference Between 21st and Twenty-First?
There is no difference in meaning. 21st is the numerical form, while twenty-first spells out the same ordinal number in words. Use whichever form suits your sentence, audience, or style guide. Both are correct.
Final Thoughts on 21th or 21st
The correct form is 21st, not 21th. Use 21st November, 21st birthday, and 21st century whenever you need the ordinal form of 21. If you prefer a date format without a suffix, “November 21” and “21 November” are also correct.
To remember the rule, check the last two digits of the number. Most numbers ending in 1 take -st, numbers ending in 2 take -nd, and numbers ending in 3 take -rd. Numbers ending in 11, 12, or 13 take -th instead.
Keep this guide handy the next time you write a date, prepare a birthday invitation, or proofread a document. Once you learn the pattern, you’ll be able to correct related mistakes such as 22th, 23th, 31th, and 11st with confidence.
