Prime Number Checker
Check if a number is a prime number instantly online. Free math utility to find out if a number is prime or composite.
Enter a number and click Check to see if it is a prime number.
Prime Number Checker & Prime Counter
A prime number is a whole number greater than 1 whose only divisors are 1 and itself (e.g., 2, 3, 5, 7, 11). Use this simple math utility — a quick prime counter for one number at a time — to instantly check whether a given number is prime or composite.
How to Use It
Type any whole number into the input field and the tool instantly reports whether it's prime or composite, showing at least one factor if it's composite.
Example
Entering 17 confirms it's prime, while entering 18 shows it's composite (divisible by 2, 3, 6, and 9).
Common Use Cases
- Verifying math homework or programming exercise results.
- Exploring number theory concepts like cryptography basics.
- Using it as a quick prime counter to check a handful of candidate numbers one by one.
- Double-checking a "is this number prime" question while coding a sieve or factorization algorithm.
FAQs
Is 1 a prime number?
No — by definition, prime numbers must be greater than 1, so 1 is neither prime nor composite.
Is this a prime counter that lists all primes in a range?
This tool checks one number at a time rather than counting or listing every prime within a range — enter each number you want checked and it instantly reports whether it's prime, plus a factor if it isn't.
Is 2 a prime number, since it's even?
Yes — 2 is prime and is the only even prime number. Every other even number is divisible by 2, which disqualifies it, but 2 itself only has the divisors 1 and 2.
What's the largest number this checker can handle?
It works reliably up to Number.MAX_SAFE_INTEGER (about 9 quadrillion) in the browser. Beyond that, floating-point precision limits make the result unreliable, so the tool shows a warning instead of a potentially wrong answer.
Why isn't 0 a prime number?
0 has infinitely many divisors (every whole number divides evenly into 0), which contradicts the definition of a prime having exactly two divisors — 1 and itself. So 0 is neither prime nor composite.