No, 21 is not a prime number. It is a composite number: its digits add up to 3, which is divisible by 3, so 21 is divisible by 3.
21 has divisors other than 1 and itself. The smallest is 3: 21 ÷ 3 = 7.
Prime factorization: 21 = 3 × 7.
| Number | Prime? |
|---|---|
| 17 | Yes |
| 18 | No, 2 × 3 × 3 |
| 19 | Yes |
| 20 | No, 2 × 2 × 5 |
| 22 | No, 2 × 11 |
| 23 | Yes |
| 24 | No, 2 × 2 × 2 × 3 |
| 25 | No, 5 × 5 |
More about 21: factors of 21 · 21 squared · square root of 21 · 21 in binary · 21 in Roman numerals.
Another number? Use the prime number checker for any value, or see the list of prime numbers.