Is 51 a prime number?
51 is not a prime number. Here is a way to check it.
The answer
No. 51 is not a prime number: 3 × 17 = 51.
How to see that 51 is not prime
Its digits add up to 5 + 1 = 6, which is a multiple of 3. That means 51 is a multiple of 3 too, so 3 is a factor.
51 has four factors: 1 and 51, plus one pair in between, 3 and 17.
Split 51 into prime factors
51 splits straight into two primes, 3 × 17, so its factor tree has only one branch.
51 in the times tables
No fact from the 2 to 12 times tables, up to × 12, makes 51. Its smallest pair, 3 × 17, needs the 17 times table, which goes past 12.
See the 17 times table, where 17 × 3 = 51 is one of the facts.
Quick checks that catch 51
- The digits add up to 6, which is in the 3 times table, so 3 goes in.
51 at a glance
Optional extra number facts:
- In words
- fifty-one
- In Roman numerals
- LI
- Odd or even
- Odd: it ends in 1
- Its factors add up to
- 72, or 21 without 51 itself. 51 is deficient: Deficient means this sum is less than the number.
The numbers either side of 51
- 50 is not prime. 50 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 50. Check 50 in full
- 52 is not prime. 52 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 52. Check 52 in full
Turn the digits round
51 backwards is 15. 15 is not prime. Its digits add up to 1 + 5 = 6, which is a multiple of 3. That means 15 is a multiple of 3 too, so 3 is a factor. Check 15 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 51?
47 below it and 53 above it, a gap of 6.
What is 51 squared?
51 × 51 = 2601.
What is the smallest number with as many factors as 51?
6. It has 4 factors, the same as 51.
Continue your number practice
Explore the diagnostic for another set of practice questions.