Is 21 a prime number?
No, 21 is not a prime number. It is divisible by 3: 21 = 3 × 7.
Is 21 prime?
Pick any whole number from 1 to 10,000.
How to work it out
- 1
A prime number has exactly two factors: 1 and itself.
- 2
It is enough to test the primes up to √21 ≈ 4.58. If none of them divides 21, it is prime.
- 3
3 divides 21 exactly, so it is not prime. Its prime factorisation is
LaTeX source of the formulas on this page
\[ 21 = 3 \times 7 \]
Testing 21 by the primes up to its square root
| Prime | Division | Result |
|---|---|---|
| 2 | 21 ÷ 2 | leaves 1 |
| 3 | 21 ÷ 3 | divides exactly (7) |
Questions
Is 21 a prime number?
No, 21 is not a prime number. It is divisible by 3: 21 = 3 × 7.
What is the next prime after 21?
23.
What is the prime number before 21?
19.
What are the factors of 21?
1, 3, 7, 21.