| ||||
|---|---|---|---|---|
| Cardinal | twenty-one | |||
| Ordinal | 21st (twenty-first) | |||
| Factorization | 3 × 7 | |||
| Divisors | 1, 3, 7, 21 | |||
| Greek numeral | ΚΑ´ | |||
| Roman numeral | XXI | |||
| Binary | 101012 | |||
| Ternary | 2103 | |||
| Senary | 336 | |||
| Octal | 258 | |||
| Duodecimal | 1912 | |||
| Hexadecimal | 1516 | |||
21 (twenty-one) is the natural number following 20 and preceding 22.
The current century is the 21st century AD, under the Gregorian calendar.