Calculators

Math

Factors

21 is the product of 3 × 7 and is significant as the traditional age of full adulthood. It's also a triangular number with 4 factors.

All Factors

The factors of 21 are:  1, 3, 7, 21

The prime factorization is: 21 = 3 x 7

Factoring of 21 Using Trial Division

Find the Factors of 21 :

• Calculate the square root of 21 which is 4.5826, and round it down to the closest whole number, 4, and use it in the next step.
• Test the integer values 1 through 4 for division into 21 where the remainder equals 0.
• We find 4 factors where the remainder of division is 0 and they are: 1, 3, 7, 21.

Factor Pairs

1 × 21 = 21
3 × 7 = 21

The factor pairs (multiples) that equal 21 when multiplied together are: (1 and 21), (3 and 7), which we found by matching up all factors of 21 which are: 1, 3, 7, 21.

Is 21 Prime or Composite?

21 is a composite number. Since it has more than two factors, we know that 21 is not prime, it is composite. Prime numbers only have two factors. They are evenly divisible by only 1 and itself.