Meet the Prime Number Checker
Check if a number is prime and get its prime factorisation, the next and previous prime, and a list of primes up to any limit. Works with very large numbers.
- Free to use
- No signup
- Nothing to install
- Runs in your browser
Illustration. Open the tool for the real thing.
What is the Prime Number Checker?
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.
3 steps to your result
Open the tool and follow along. Nothing to install.
What you get
Works on very large integers with exact arithmetic.
Shows the full factorisation with exponents.
The nearest previous and next prime numbers.
Generate all primes up to 100,000.
AZRS tools are free to open and use.
It opens in your browser, so there is nothing to download.
Used for
Pick one to see how it fits.
Programmers and puzzle solvers
Test numbers quickly, for example in Project Euler problems.
Try it freeHow it compares
A plain look at this tool next to the usual alternative.
| This calculator | Desktop software | |
|---|---|---|
| Install | Nothing to install | Usually an app or program to install |
| Account | No signup | Depends on the product |
| Cost | Free to use | Free or paid, depends on the product |
| Where it runs | In your browser tab | On your computer |
Questions people ask
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.
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