Number
21 in Roman numerals
21 is written XXI
Type any other number or numeral below to convert it.
Above 3,999? Type the barred part in brackets, e.g. (IV)CC = 4,200.
How XXI is built
Roman numerals are written one decimal place at a time, largest first.
- XXtens · 2X written 2 times: 2 × 10 = 20.
- Iones · 1I is 1.
20 + 1 = 21
No subtraction is needed here: every letter is the same size or smaller than the one before it, so you simply add them up.
About the number 21
- 21 is odd and can be split into prime factors: 3 × 7, or in Roman numerals III × VII.
- XXI has 3 letters. 907 of the numbers up to 1,000 have longer numerals.
- As a day of the month, XXI is the 21st. Write a date
Counting from 21 to 30
Every number from 21 to 30 starts XX (20); only the ending changes.
| Number | Roman numeral |
|---|---|
| 21 | XXI |
| 22 | XXII |
| 23 | XXIII |
| 24 | XXIV |
| 25 | XXV |
| 26 | XXVI |
| 27 | XXVII |
| 28 | XXVIII |
| 29 | XXIX |
| 30 | XXX |