A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number.
For example, 5 is prime because the only ways of writing it as a product are 1 × 5 or 5 × 1, whereas 6 is composite because it is the product of 2 × 3 (or 3 × 2).
The first few prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, ...
Click on numbers to check if they are prime or composite. The calculator will show you all the factors of each number.
Classification: Unknown
Factors:
The Sieve of Eratosthenes is an ancient algorithm for finding all prime numbers up to a specified limit. Watch how it works by crossing out multiples of each prime number.
Test your understanding of prime numbers with these challenges!