How to write 21 in Roman numerals
Roman numerals are built place by place, from the largest value to the smallest. For 21 the places are 20 (tens), 1 (ones).
- Write 20 as
XX. - Write 1 as
I.
Joined together, XX + I gives XXI.
21 digit by digit
| Place | Value | Numeral | Rule used |
|---|---|---|---|
| tens | 20 | XX | the symbol repeated twice |
| ones | 1 | I | one symbol |
Check XXI by reading it back
Reading XXI from left to right and adding the pieces: XX = 20, I = 1. Together that is 20 + 1 = 21, so XXI and 21 match.
Mistakes to avoid with XXI
- Never repeat a symbol more than three times in a row. When a fourth would be needed, the subtractive form takes over.
- Only I, X and C may be placed before a larger symbol to subtract, so XXI never contains forms such as VX or LC.
- Read XXI as one number, not letter by letter: add the values left to right, except where a smaller symbol sits before a larger one.
Facts about the number 21
- 21 is an odd number and composite, with prime factors 3 x 7.
- In binary 21 is
10101and in hexadecimal it is15. - Written out, 21 is "twenty-one".
- The digits of 21 add up to 3, so it is divisible by 3.
Where you will see 21 in Roman numerals
Academic papers reference the twenty first century using XXI for the number 21. Major sporting events also adopted it for modern edition numbering. Institutions select this three letter combination to formally designate the current era, distinguishing it clearly from previous centuries in official records and public communications.
Numbers near 21
FAQ
What is 21 in Roman numerals?
21 is written XXI in Roman numerals. It is built from XX (20), I (1).
How do you write 21 in lowercase Roman numerals?
Lowercase Roman numerals use the same rules, so 21 is xxi. Lowercase is common for page numbers in a book's preface and for outline levels.
How do I convert any number to Roman numerals?
Split the number into thousands, hundreds, tens and ones, write each part with the symbols I, V, X, L, C, D and M, then join them from largest to smallest. Or paste it into the Roman numeral converter, which also converts back.
More roman numerals
- 20 (XX): previous page
- 22 (XXII): next page
- Roman numerals chart: the full list