Prime Number Checker
Check whether a number is prime. For composite numbers, shows the prime factorisation. Also reports the previous prime and next prime. Uses deterministic Miller-Rabin for very large numbers.
What powers the Prime Number Checker
Primality test (trial division + Miller-Rabin)
Prime factorisation for composites
Previous / next prime
Handles numbers beyond JS safe integer range
Why the Prime Number Checker is different
Browser-native
The Prime Number Checker runs entirely in your browser. Input is processed locally — never uploaded, never logged, never cached anywhere outside your device.
No artificial limits
No daily quotas, no character ceilings, no "upgrade for more" walls. Every feature is the complete feature — the same on the first use as the thousandth.
Production-grade quality
Built to the same engineering bar as paid SaaS tools — accurate algorithms, audited logic, responsive design and accessibility-tested interactions.
Common use contexts
- Number theory homework
- Cryptography exploration
- Programming interview prep
Private and secure
Zero upload
All processing happens in your browser. Input is never transmitted, logged or cached.
Works offline
Once the page loads, the tool runs without an internet connection. No network calls happen during use.
No tracking
No accounts, no cookies for tool state. Only aggregate analytics count visits at the page level.