odd, composite, Fibonacci, triangular · 21st positive integer
The number 21
21 is an odd composite number. It's known as the legal drinking age in the United States and the perfect blackjack score. It is the 21st positive integer.
Quick reference
| Prime factorization | 3 × 7 |
| Divisors | 1, 3, 7, 21 |
| Binary | 10101 |
| Hexadecimal | 0x15 |
| Roman numeral | XXI |
| Digit sum | 3 |
| Properties | odd, composite, Fibonacci, triangular |
Why 21 is notable
The legal drinking age in the United States and the perfect blackjack score.
Frequently asked
Is 21 a prime number?
No. 21 is composite. Its prime factorization is 3 × 7.
What are the factors of 21?
The factors (divisors) of 21 are 1, 3, 7, 21.
What is 21 in binary?
21 in binary is 10101. In hexadecimal it's 15.
What is 21 as a Roman numeral?
21 in Roman numerals is XXI.
What's the digit sum of 21?
Adding the digits of 21: 2 + 1 = 3.