Prime Number Checker
Check whether a number is prime, see its factors if it isn't, and find the nearest primes.
About the Prime Number Checker
Enter a whole number to test whether it's prime. The calculator gives the answer, the factors if it isn't prime, and the primes either side.
A prime number has exactly two factors: 1 and itself. To test a number you only need to try dividing by primes up to its square root; if none divides it, it's prime.
The formula
n is prime if no prime p ≤ √n divides n
The first 25 primes
| Range | Primes |
|---|---|
| 1–50 | 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 |
| 51–100 | 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 |
Worked example
97
- √97 ≈ 9.85, so try 2, 3, 5 and 7
- None divides 97
- 97 is prime; the neighbouring primes are 89 and 101
Questions people ask
Is 2 prime?
Yes, and it's the only even prime.
How many primes are there?
Infinitely many, as Euclid proved more than 2,000 years ago.