Prime Number Checker
Check if a number is prime and find the nearest primes above and below it.
How to use
Enter a whole number and click Check. Everything runs in your browser — nothing is uploaded or stored.
Frequently asked questions
Is 1 a prime number?
No — primes start at 2.
How large a number can it test?
Reliably up to about 10¹⁵ with trial division.
☕
ToolZap is free forever
Every tool runs 100% in your browser — no uploads, no signups. If a tool saved you time, consider supporting us with a small donation.
💛 Donate via PayPal