Prime Number Checker
MATHCheck if a number is prime and find all its factors.
ℹ️ About Prime Number Checker
A prime number is a whole number greater than 1 that can only be divided evenly by 1 and itself. They are the building blocks of all other numbers and the backbone of modern encryption. This checker tells you instantly whether a number is prime, and if it is not, it lists the factors that divide it — useful for homework, simplifying fractions, or just satisfying curiosity about a big number.
📖 How to Use Prime Number Checker
1
Enter a whole number
Type any positive integer you want to test.
2
Check the result
See immediately whether it is prime or composite.
3
Review the factors
For composite numbers, the divisors are listed.
4
Try another
Test as many numbers as you like — it is instant and free.
💡 Examples
Prime
INPUT
17→
OUTPUT
Prime — divisible only by 1 and 17Looks prime but isn’t
INPUT
91→
OUTPUT
Not prime — 7 × 13Composite
INPUT
100→
OUTPUT
Not prime — factors 2, 4, 5, 10, 20, 25, 50📊 Prime Numbers up to 100
There are 25 prime numbers below 100:
| Range | Primes |
|---|---|
| 1–50 | 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 |
| 51–100 | 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 |
Divisibility shortcuts: ÷2 if even · ÷3 if the digits sum to a multiple of 3 · ÷5 if it ends in 0 or 5.
🚀 Pro Tips
✓2 is the only even prime number — every other even number is divisible by 2.
✓1 is not prime (and not composite); primes are defined as greater than 1.
✓To test by hand, you only need to check divisors up to the square root of the number.
✓Quick divisibility checks: even numbers divide by 2; digit sums divisible by 3 divide by 3; numbers ending in 0 or 5 divide by 5.
✓Numbers like 91 (7×13) and 51 (3×17) look prime but aren’t — always verify.