Prime Number Checker & Factorization
Enter a whole number to check if it's prime and see its prime factors.
About the Prime Number Checker & Factorization
The Prime Checker tells you whether a whole number is prime and, if it is not, shows its complete prime factorisation. Students use it for number theory homework, and programmers use it to test ideas.
A prime number is greater than 1 and has no divisors other than 1 and itself. Every other whole number above 1 is composite and can be written as a unique product of primes, such as 84 = 2 × 2 × 3 × 7. The tool divides by candidate factors up to the square root of the number, which is fast enough for numbers up to about a trillion. For much larger numbers, specialised software is needed.
How to use it
- Enter a whole number of 2 or more.
- Read whether it is prime or composite.
- See the prime factors listed as a product.
- Try another number.
Tips and good to know
- Even numbers above 2 are never prime.
- A number is divisible by 3 if its digits add up to a multiple of 3.
- The number 1 is neither prime nor composite.
Frequently asked questions
Is 2 prime?
Yes, it is the only even prime.
How large a number can I test?
Up to about one trillion.
Why is 1 not prime?
Primes are defined as having exactly two distinct divisors.