Prime Number Checker
Is it prime? Factorise big numbers instantly · free, no signup
Free Prime Number Checker and Prime Factorization Calculator
A prime number is a whole number greater than 1 whose only divisors are 1 and itself. Primes are the building blocks of all whole numbers and underpin modern cryptography. This tool tells you instantly whether a number is prime and, if not, breaks it into its prime factors.
It uses fast deterministic tests for large numbers and Pollard’s rho method to factorise, so it can handle numbers with many digits that would take a calculator a long time. You also get the previous and next prime and can generate a list of every prime up to a limit you choose.
Key features
Instant primality test
Works on very large integers with exact arithmetic.
Prime factorisation
Shows the full factorisation with exponents.
Neighbour primes
The nearest previous and next prime numbers.
Prime list
Generate all primes up to 100,000.
How to use it
- Enter a whole number greater than 1.
- See whether it is prime and its prime factors.
- Look at the previous and next primes.
- Optionally list all primes up to a limit.
Worked example
Example
97 → prime 91 → not prime = 7 × 13 600851475143 = 71 × 839 × 1471 × 6857 2305843009213693951 (2⁶¹ − 1) → prime
Who uses this tool
Students
Check homework on factors, primes and factorisation.
Programmers and puzzle solvers
Test numbers quickly, for example in Project Euler problems.
Curious minds
Explore primes and famous large primes.
Tips for the best results
- Remember that 1 is not prime and 2 is the only even prime.
- For a quick check, a number ending in 0, 2, 4, 5, 6 or 8 (except 2 and 5) is not prime.
- Large semiprimes (two big primes multiplied) can take a little longer.
- Use the list for a quick reference of small primes.
Common mistakes to avoid
- Assuming odd numbers are prime — 9, 15, 21 and 25 are not.
- Entering decimals or negative numbers.
- Expecting instant factorisation of enormous numbers with only large prime factors.
Why use AZRS QuickFix?
It is 100% free, needs no signup and has no watermark or usage limits. The tool runs in your browser, so what you type stays on your device, and it works on phones, tablets and desktops. New tools are added every week — bookmark this page or browse the full QuickFix toolbox.
Frequently asked questions
Is 1 a prime number?
No. By definition a prime has exactly two distinct divisors.
What is prime factorisation?
Writing a number as a product of prime numbers, for example 60 = 2² × 3 × 5.
How large a number can I test?
Very large numbers work; extremely big numbers with huge prime factors may take longer to factorise.
Which even number is prime?
Only 2.
What is the largest known prime?
Mersenne primes hold the record; they have tens of millions of digits.
Is it free?
Yes, free and private in your browser.