21 in Roman numerals
XXI
The number 21 is written XXI in Roman numerals.
How XXI breaks down
| Symbols | Value |
|---|---|
XX | 2 × 10 = 20 |
I | 1 |
XXI | 21 |
Every symbol here is written in descending order and simply added together — no subtractive pairs are involved.
Nearby numbers
| Number | Roman numeral |
|---|---|
| 16 | XVI |
| 17 | XVII |
| 18 | XVIII |
| 19 | XIX |
| 20 | XX |
| 21 | XXI |
| 22 | XXII |
| 23 | XXIII |
| 24 | XXIV |
| 25 | XXV |
| 26 | XXVI |
The seven symbols
I | 1 | C | 100 |
V | 5 | D | 500 |
X | 10 | M | 1,000 |
L | 50 |
21 in other notations
| Roman | XXI |
| Binary | 10101 |
| Hexadecimal | 15 |
| Octal | 25 |
| Written out | twenty-one |
Writing it by hand
Work down through the symbol values, taking as many of each as will fit before moving to the next. For 21:
2 × X = 20 remaining: 1
I = 1 remaining: 0The result reads XXI — 3 characters, against 2 digits in Arabic numerals.
Frequently asked questions
What is 21 in Roman numerals?
XXI
How do you read XXI?
Work left to right, adding each symbol — except where a smaller symbol precedes a larger one, in which case it is subtracted. XX is 20, I is 1, giving 21 in total.
How do you write 21 in words?
twenty-one.
Why is there no zero?
Roman numerals have no symbol for zero. The concept reached Europe only with Hindu-Arabic numerals, centuries after the system was in use — which is a large part of why it was eventually replaced for arithmetic.