Is 21 a prime number?
21 is not a prime number. Here is a way to check it.
The answer
No. 21 is not a prime number: 3 × 7 = 21.
How to see that 21 is not prime
Its digits add up to 2 + 1 = 3, which is a multiple of 3. That means 21 is a multiple of 3 too, so 3 is a factor.
21 has four factors: 1 and 21, plus one pair in between, 3 and 7.
Split 21 into prime factors
21 splits straight into two primes, 3 × 7, so its factor tree has only one branch.
21 in the times tables
21 turns up in 2 different times tables, the 3 and 7: 3 × 7 and 7 × 3.
Quick checks that catch 21
- The digits add up to 3, which is in the 3 times table, so 3 goes in.
21 at a glance
Optional extra number facts:
- In words
- twenty-one
- In Roman numerals
- XXI
- Odd or even
- Odd: it ends in 1
- Triangular number
- 1 + 2 + … + 6 = 21, a triangle of counters with 6 on the bottom row
- Its factors add up to
- 32, or 11 without 21 itself. 21 is deficient: Deficient means this sum is less than the number.
The numbers either side of 21
- 20 is not prime. 20 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 20. Check 20 in full
- 22 is not prime. 22 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 22. Check 22 in full
Turn the digits round
21 backwards is 12. 12 is not prime. 12 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 12. Check 12 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 21?
19 below it and 23 above it, a gap of 4.
What is 21 squared?
21 × 21 = 441.
What is the smallest number with as many factors as 21?
6. It has 4 factors, the same as 21.
Continue your number practice
Explore the diagnostic for another set of practice questions.