Factors of 21
There are 4 factors of 21 — the full list is below, with factor pairs, prime factorization, and divisibility notes.
Factors of 21
1, 3, 7, 21
Factor pairs of 21
- 1 × 21
- 3 × 7
Prime factorization of 21
21 = 3 × 7
Number facts
- 21 is a composite number.
- 21 has 4 factors.
- 21 is not a perfect square.
- The sum of all factors of 21 is 32.
Divisibility rules that apply to 21
- 21 is divisible by 3 because the sum of its digits (3) is divisible by 3.
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How the factors of 21 are calculated
To find the factors of 21, every whole number from 1 up to the square root of 21(about 4.58) is tested for even division. 21 is odd, and it divides evenly by 3, 7. Written as a product of primes, 21 = 3 × 7, which confirms every factor above is a combination of those prime building blocks.
Frequently asked questions about 21
How many factors does 21 have?
21 has 4 factors: 1, 3, 7, 21.
Is 21 a prime number?
No. 21 is composite because it has factors other than 1 and itself.
What are the factor pairs of 21?
The factor pairs of 21 are 1 × 21, 3 × 7.
What is the prime factorization of 21?
The prime factorization of 21 is 3 × 7.