Prime Number Checker
Check if any whole number is prime with instant results.
It has exactly two factors: 1 and itself.
About this Tool
The Prime Number Checker tests whether any whole number up to one billion is prime, lists all of its factors, and shows the nearest primes above and below. Primality is tested instantly in your browser with an optimized trial-division check. It is a handy companion for math homework, coding challenges, and cryptography experiments.
Common Use Cases
Math Homework
Verify whether numbers like 97 or 143 are prime and list their factors to show full working for divisibility exercises.
Coding Challenges
Cross-check outputs of your own isPrime function or sieve implementation against instant ground-truth results while practicing algorithms.
Cryptography Learning
Explore how primes sit among composites and how far apart neighbouring primes are when studying RSA key concepts.
Puzzle & Game Design
Pick prime numbers for puzzle answers, game seeds, or hash-table sizes where prime moduli reduce clustering.
Pro Tips
- !
Check Small Divisors First
A number ending in an even digit or 5 (beyond 2 and 5 themselves) is never prime. Quick checks for 2, 3, and 5 rule out most composites instantly.
- !
Only Test Up to the Square Root
If no divisor is found up to √n, none exists above it either. That is why even numbers near a billion check in milliseconds.
- !
1 Is Not Prime
By definition a prime has exactly two distinct factors. The number 1 has only one factor, so it is neither prime nor composite.
FAQs
Was this calculator helpful?