Is 21 a Prime Number?
Is 21 prime?
No
21 = 3 × 7 (divisible by 3).
Why
21 is composite because it is divisible by 3 (21 = 3 × 7), so it has factors other than 1 and itself.
A prime number is a whole number greater than 1 whose only factors are 1 and itself. To check 21, test whether any prime up to √21 divides it evenly — if none do, it is prime.
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The prime factorization calculator tests any number and shows its prime factors and all divisors.
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Frequently asked questions
Is 21 a prime number?+
No, 21 is composite because it is divisible by 3 (21 = 3 × 7), so it has factors other than 1 and itself.
What is 21 divisible by?+
21 is divisible by more than just 1 and itself. Its prime factorization is 21 = 3 × 7.