Prime Number Checker — Test, Factorise and List Primes
Check whether a number of any size is prime, see its prime factorisation and the neighbouring primes, or list all primes in a range.
Free, with no sign-up and no watermark. The file is processed in your browser and never uploaded.
Find out whether a number is prime — even one with hundreds of digits — see its prime factorisation, number and sum of divisors, and the previous and next primes. You can also list every prime in a range.
Step by step
- Enter a number: Type any positive whole number in Number to check.
- Read the verdict: Prime, composite, or “almost certainly prime” for extremely large numbers.
- See the details: Composite numbers show their prime factorisation and divisor counts.
- List a range: Set From and To to list primes (span up to 10 million) and Download list.
Handy for
Maths homework
Check primality and factorise numbers quickly.
Programming
Get prime lists for testing code.
Cryptography learning
Explore large primes and factorisation.
Puzzles
Find the next prime after any number.
Good to know
Numbers below 3.3 × 10²⁴ are proven prime with a deterministic test.
Factorising numbers with two very large prime factors can take a long time; the tool stops after a few seconds and says so.
Questions
What does “almost certainly prime” mean?
For numbers above 3.3 × 10²⁴ the tool runs the Miller–Rabin test with 13 fixed and 37 random bases. The chance that a composite number passes all of them is below 1 in 10²².
Is 1 a prime number?
No. Primes are whole numbers greater than 1 with exactly two divisors.
Is anything uploaded?
No. Everything runs in your browser, and anything you save (decks, timetables, results) stays in this browser’s local storage on your device.
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