Is 1 a prime number?

A prime number is a whole number greater than 1 that can be divided evenly only by 1 and by itself. Examples include 2, 3, 5, 7, 11 and 13. Numbers with more than two factors, like 4, 6 and 9, are called composite numbers. The number 1 belongs to neither group.

For a long time, some mathematicians did count 1 as prime. It was left out because including it would break an important rule called the fundamental theorem of arithmetic, which says every whole number greater than 1 can be written as a product of primes in exactly one way. If 1 were prime, you could add as many 1s as you like, so the way would no longer be unique.

Fun facts about primes

Two is the only even prime number. Euclid proved more than 2,000 years ago that there are infinitely many primes. The largest known primes are Mersenne primes with tens of millions of digits, found by computer searches.

Did You Know?

Prime numbers protect your data online. Common encryption systems rely on the fact that multiplying two huge primes is easy, but working backward to find them from the result is extremely hard.