Names of 21
| Nominal | 21 |
| Cardinal | twenty-one |
| Ordinal | 21st |
| This number as US currency | twenty-one dollars |
| Is 21 an odd number? | 21 is an odd number |
| Is 21 an even number? | 21 is NOT an even number |
| Is 21 a palindrome? | 21 is NOT a palindrome number |
| Is 21 a triangle number? | 21 is a triangle number |
21 in other Bases
| Base 2 (Binary) | 101012 |
| Base 3 (Ternary) | 2103 |
| Base 4 (Quaternary) | 1114 |
| Base 5 (Quinary) | 415 |
| Base 6 (Senary) | 336 |
| Base 8 (Octal) | 258 |
| Base 12 (Duodecimal) | 1912 |
| Base 16 (Hexadecimal) | 1516 |
| Base 20 (Vigesimal) | 1120 |
Primes and Factors tests for 21
| Is 21 a Prime Number? | 21 is NOT a prime |
| Is 21 a perfect number? | 21 is NOT a perfect number |
| Is 21 a Mersenne Prime? | 21 is NOT a Mersenne prime |
| Is 21 a Fermat Prime? | 21 is NOT a Fermat prime |
| Is 21 a Fibonacci Prime? | 21 is NOT a Fibonacci prime |
| Is 21 a Partition Prime? | 21 is NOT a Partition prime |
| Is 21 a Pell Prime? | 21 is NOT a Pell prime |
| Prime Factors | Prime factors of 21 |
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21 in other Numerals
| 21 in Roman Numeral | XXI |
| 21 in Chinese Numeral | 貳拾壹 |
| 21 in Egyptian Numeral | 𓎇𓏺 |
| 21 in Babylonian Numeral |  |
Roots and stuff for 21
| Is 21 a square number? | 21 is NOT a square number |
| Is 21 a cube number? | 21 is NOT a cube number |
| Square root of the number 21 | 4.5825756949558 |
| Cube root of the number 21 | 2.7589241763811 |
Recreational math for 21
| Number 21 reversed | 12 |
| Sum of the digits | 3 |
| No of digits | 2 |