Prime number checker

Check whether a number is prime and see its prime factors and the nearest primes.

🔒 Everything runs in your browser. Nothing you choose or type is uploaded. How to verify

How to use it

Enter a whole number.
Read the answer, which updates as you type.
Use the nearest primes or factors for your task.

Handy for

Maths homework, puzzles and programming exercises.

Good to know

Composite numbers show their prime factorization, such as 100 = 2 × 2 × 5 × 5.

Privacy: where the work happens

Everything you enter or choose is processed by JavaScript in your browser and never sent anywhere. Only the page code, and any library this tool needs, is downloaded from cdnjs or jsDelivr.

Questions

How large a number can it check?

Up to 1,000,000,000,000.

Is 1 prime?

No. A prime has exactly two divisors, and 1 has only one.