Prime Number Checker
Check if a number is prime, list its factors and prime factorisation, and list all primes in a range.
About Prime Number Checker
This prime number checker online tells you instantly whether a whole number is prime. It also shows the nearest primes above and below, every factor of the number and its prime factorisation, so you can see why a number is or is not prime.
Enter a range to list every prime number in it, for example the prime numbers from 1 to 100 for a school exercise. It is useful for students, teachers and anyone testing numbers while learning to write a prime number check in code. The test runs in your browser, is free and needs no sign-up.
How to use Prime Number Checker
- Enter a whole number to test.
- See whether it is prime, its nearest primes, all factors and prime factorisation.
- Enter a range below to list every prime in it.
Frequently asked questions
Is 1 a prime number?
No. A prime has exactly two distinct factors, 1 and itself. 1 has only one factor, so it is neither prime nor composite.
Is 2 the only even prime?
Yes. Every other even number is divisible by 2, so it has at least three factors.
How are primes in a range found?
With the Sieve of Eratosthenes: cross out multiples of each prime up to √(end); what remains are primes. Ranges are limited to 2,000,000 so it stays fast.
How do I check if a number is prime?
Try dividing it by every whole number from 2 up to its square root. If none divides it exactly, it is prime; the checker does this for you instantly.
How many prime numbers are there from 1 to 100?
There are 25 primes between 1 and 100, starting 2, 3, 5, 7, 11 and ending with 97. Enter the range 1 to 100 to see them all.
What is prime factorisation?
It is writing a number as a product of prime numbers, such as 84 = 2 × 2 × 3 × 7.
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