Factors of 21

All factors of 21

Here are all 4 factors of 21, listed from smallest to largest.

Factor pairs of 21

These 2 pairs of whole numbers each multiply to give 21.

Prime factorization of 21

Dividing 21 by the smallest prime at each step gives its prime factorization:

The distinct prime factors of 21 are 3, 7. To factorize any other number, use the prime factorization calculator.

Factor tree of 21

A factor tree shows the same prime factorization visually. Each composite branch splits until every endpoint is a prime number.

Properties of 21

What makes 21 interesting

• 21 is a composite number, meaning it has more than two factors. It can be written as a product of smaller whole numbers in 2 different ways.
• 21 is a deficient number: its proper factors sum to 11, which is 10 less than 21 itself.
• 21 appears in the Fibonacci sequence, where each number is the sum of the two before it.
• 21 is a triangular number — it is the sum of the first 6 whole numbers (1 + 2 + … + 6 = 21).

How to find the factors of 21 yourself

Divide 21 by each whole number starting from 1. Whenever the division leaves no remainder, both the divisor and the quotient are factors. You only need to test up to √21 ≈ 4.58, because every factor above that pairs with one below it.

Reading both columns gives every factor: 1, 3, 7, 21. The factor calculator does this automatically for any number.