Check whether a whole number is prime and, when it is not, see its smallest factor.
Try an example
Load a ready-made sample into this tool and run it. Every result you see is calculated live by the tool itself — nothing here is a pre-written answer.
Sample: input: 29
How to Use
- Enter a whole number.
- Click Run.
- Read whether it is prime or its smallest factor.
What this prime number checker does
This prime number checker tests a single whole number and reports whether it is prime — divisible only by 1 and by itself — or composite. When the number is not prime, it also shows the smallest factor that divides it, so you see immediately why it failed the test rather than simply being told that it did. The same quick check serves homework, puzzles and programming exercises alike.
How the check works
The test uses trial division: the number is divided by candidate divisors in turn, looking for one that leaves no remainder. Finding any divisor other than 1 and the number itself proves the number composite, and the first one found is its smallest factor. A number that survives every test up to its square root is prime.
Good to know
Zero and 1 are not prime — by definition a prime needs exactly two divisors, and 1 has only one. Very large numbers may take a moment to check, because trial division tests many candidates. The checker works on one whole number at a time, not on ranges of numbers or lists.
Frequently asked questions
Is 1 a prime number?
No. A prime needs exactly two distinct divisors — 1 and itself as separate values. The number 1 has only one divisor, so it is neither prime nor composite, and the checker reports it as not prime.
What does the smallest factor tell me?
It is the smallest number besides 1 that divides yours exactly. It proves the number composite and gives a starting point if you want to break the number down further by hand.
Why do large numbers take longer to check?
Trial division tests candidates one by one until it finds a factor or exhausts possibilities up to the square root. Bigger numbers mean more candidates, so very large checks take longer.
How to use Prime Number Checker
- Enter a whole number.
- Click Run.
- Read whether it is prime or its smallest factor.
Limitations
Checking uses trial division, so very large numbers may take a moment; 0 and 1 are not prime.