21th or 21st – Which Spelling Is Correct?

What’s the correct spelling and form for the number 21 in its ordinal form? Should we use “21th” or “21st”?

We want to know what’s the correct form, and why.

21th or 21st – Which Spelling Is Correct?

“21st” is the proper spelling and form for the number 21 in its ordinal form. The number 1 is an irregular ordinal number and the correct form is “First”. The suffix “-st” follows most numbers that end with 1 – that’s the case with “21st”. “21th” is incorrect and unacceptable.

21th or 21st

Take a look at the examples below:

  • Guys, save the date: the wedding will be on October 21th. (incorrect)
  • Guys, save the date: the wedding will be on October 21st.

You’re probably aware that most ordinal numbers end, indeed, with the suffix “-th” and that reflects also on its numeral form. That’s why some people assume that “21th” would be the correct form to use.

It’s important to keep in mind, however, that in the English language we have regular and irregular forms – which are named “irregular” because they’re different and follow a different rule. That’s what happens with the form “21st”.

Instead of following the rule that governs most ordinal numbers, “21st” follows the rule for number 1, which ordinal form is “First” or “1st”. Always use “21st”, as a number, as a word, and even verbally. That’s the correct way to express this ordinal number.


“21th” is an incorrect form for the ordinal form of the number 21 and it should be avoided. Although “-th” is a common suffix for ordinal numbers, some ordinals are considered to be irregular. Those, have a different suffix attached to them.

When it comes to the number 1 and most of the numbers that end with 1, the proper suffix to use is “-st”. The number one is the “First” ordinal number and all numbers with one follow suit and go with “-st”.

Is “21th” a word? The answer is no. “21th” isn’t a word or an acceptable form, according to The Cambridge Dictionary. We should all avoid using it.

Let’s see some examples that include the incorrect form “21th”, followed by a corrected version of the sentence:

  1. It’s Cam’s 21th birthday! (incorrect)
  2. It’s Cam’s 21st birthday!
  1. The event will be held on the 21th of the month. (incorrect)
  2. The event will be held on the 21st of the month.
  1. Frank’s birthday is on July 21th. (incorrect)
  2. Frank’s birthday is on July 21st.


“21st” is the correct form to indicate the ordinal number between 20th and 22nd. “21th” is an irregular ordinal number, because it ends with 1. The number one, as an ordinal number, is “First” – and the suffix used to indicate that is “-st”. That’s why we use “1st”, “21st”, “31st”, etc.

The Cambridge Dictionary forwards the reader who searches for “21st” to the written version of it, which is “Twenty-First”. It states that “Twenty-First” is “21st written as a number”. Therefore, we can infer that “21st” can be defined as “twenty-first written as a number”.

Take a look at some examples below:

  1. My dad gave me a car for my 21st birthday.
  2. Technology is something unique to the 21st century.
  3. It’s September 21st.
  4. On the 21st of December, Sasha will be traveling to England.
  5. It’s David’s 21st wedding anniversary.
  6. I’ll be back to school on August 21st.

Which Is Used the Most?

Which one of those forms is used more often, “21th” or “21st”? Let’s find out by taking a look at the graph from Google Ngram Viewer below.

21th or 21st usage

“21st”, which is the correct form, is the one used the most by people in their speeches. The graph indicates it’s the preferred form, which people apply more often.

“21th” is rarely used and appears at the bottom of the graph. That indicates that people tend to avoid it, as they should because it’s an incorrect form for this ordinal number.

Final Thoughts

The ordinal form of the number 1 is “First” because it’s an irregular ordinal number. The suffix “-st” is the one used in this case, so we have “1st”. This is the rule that follows most numbers that end with 1 – such as “21st”, for example. “21th” is a misspelling.