Prime Number Checker
Is a number prime? Instant yes/no with factor if composite.
See how it works
Watch a quick simulation of this tool in action — no file needed.
How to use Prime Number Checker
Follow these steps — everything happens instantly in your browser.
- 1
Step 1. Enter any positive integer (up to ~10¹⁵).
- 2
Step 2. The tool instantly says whether it is prime.
- 3
Step 3. If composite, the smallest prime factor is shown.
Why use this tool
Private by design
All processing happens locally in your browser. Open DevTools → Network and you'll see zero file uploads. Ever.
Instant
No upload wait, no queue, no server round-trip. Results appear the moment you drop a file.
Free & unlimited
No accounts, no watermarks, no daily caps. Use it as many times as you like, forever.
Works everywhere
Desktop, tablet or phone — every tool is responsive and installable as a PWA for offline use.
Frequently asked questions
Everything you might want to know about prime number checker.
Prime Number Generator
List the first N primes — or primes up to a limit.
Prime Factorization Calculator
Break any integer into its prime factors — 2² × 3 × 5, etc.
GCF & LCM Calculator
Greatest common factor and least common multiple of any integers.
Fibonacci Generator
Generate the first N Fibonacci numbers — arbitrary precision.