Feedback form

Is 21 a prime number?

Prime Number Checker

Is 21 a prime or composite number?

Answer

Number 21 is not a Prime

21 = 3 × 7

21 has 4 divisors — 1, 3, 7 and 21; a prime has exactly two: 1 and itself.

Primes up to √21

4² = 16 ≤ 21 < 25 = 5², so it is enough to try primes up to 4

÷2 does not divide
÷3 divides

21 ÷ 3 = 7 — 3 divides 21, so 21 is composite

Nearest primes: 19 and 23

Found an error in the calculation or the information?

Related Calculations

See Also

Factors of a Number - List all Factors and Factor Pairs of a Number
Prime Factorization - Prime Factorization Calculator
Prime Numbers List - List of all Prime Numbers - how many Prime numbers are between

Share This Calculation

Print

https://calculat.io/en/number/is-prime-number/21

<a href="https://calculat.io/en/number/is-prime-number/21">Is 21 a prime number? - Calculatio</a>

Facebook

Twitter

Telegram

WhatsApp

Viber

Email

Is 21 a prime number? Is 21 a prime number? 21 is a composite number. Check if any number is prime or composite with our free online prime checker.

https://calculat.io/en/number/is-prime-number/21/generated.jpg

Prime numbers list

Number	Is Prime
6	No: 2 × 3
7	Yes
8	No: 2³
9	No: 3²
10	No: 2 × 5
11	Yes
12	No: 2² × 3
13	Yes
14	No: 2 × 7
15	No: 3 × 5
16	No: 2⁴
17	Yes
18	No: 2 × 3²
19	Yes
20	No: 2² × 5
21	No: 3 × 7
22	No: 2 × 11
23	Yes
24	No: 2³ × 3
25	No: 5²
26	No: 2 × 13
27	No: 3³
28	No: 2² × 7
29	Yes
30	No: 2 × 3 × 5
31	Yes
32	No: 2⁵
33	No: 3 × 11
34	No: 2 × 17
35	No: 5 × 7

FAQ

Is 21 a prime or composite number?

Number 21 is not a Prime

How many factors are in a number 21?

4

How to check if 21 is prime?

Divide 21 by the primes in order: 4² = 16 ≤ 21 < 25 = 5², so it is enough to try primes up to 4. The first one that divides it is 3: 21 ÷ 3 = 7, so 21 is composite.