Prime number checker
Enter a whole number to find out if it is prime, meaning its only divisors are 1 and itself. The test is exact, not probabilistic, within the tool range.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter number.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Miller-Rabin test with the prime bases 2 to 37, which is deterministic for every number below 3.3 × 10^24. A number is composite if it has a divisor between 2 and its square root.
Example
Inputs: Number = 91. Result: Result: Composite; Smallest prime factor: 7; Next prime: 97.
Limits and caveats
- Accepts whole numbers from 0 to 10^15.
- By definition 0 and 1 are neither prime nor composite.
Questions about this tool
Is 1 a prime number?
No. A prime has exactly two distinct divisors, 1 and itself, and 1 has only one divisor.
Is 2 prime?
Yes, and it is the only even prime, since every other even number is divisible by 2.
Why is 91 not prime?
Because 91 = 7 × 13. The tool shows the smallest prime factor so you can see the reason at once.