Number reference
5 explained.
5: five, prime, fibonacci number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- five
- Classification
- prime
- Factors
- 1, 5
- Prime factorization
- 5
- Square root
- 2.236068
- Cube root
- 1.709976
- Binary
- 101
- Octal
- 5
- Hexadecimal
- 5
- Roman numeral
- V
- Digit sum
- 5
- Digital root
- 5
What makes 5 notable
- Prime. 5 is prime: it has exactly two divisors, 1 and itself, so it cannot be split into equal whole-number groups any other way.
- Fibonacci number. 5 appears in the Fibonacci sequence, where each term is the sum of the two before it.
5 in everyday use
Five is the number of fingers on a hand, which is why tally marks are bundled in fives and why Roman V is half of X. It is the atomic number of boron, the count of Olympic rings, and the number of players a basketball team fields. English has five vowel letters, a starfish has five arms, and the five-day working week has been the norm in industrial countries since the 1930s. A pentagon has five sides; the Pentagon in Virginia takes its name from that shape.
Five is the third prime and the only prime that ends in 5, since every other number ending in 5 divides by it. It is a Fibonacci number, following 3 and preceding 8, and the hypotenuse of the 3-4-5 triangle, the simplest right triangle with whole-number sides. Written as 1 + 4, it is the smallest prime that is a sum of two squares. As a Fermat prime (2² + 1), it lets a regular pentagon be constructed with ruler and compass.
Nearby numbers
How to read this page
Because 5 is prime, the factor list stops at 1 and 5 itself, so it cannot be shared into equal whole-number groups. That is also what makes it useful as a building block: every larger number with 5 in its prime factorisation inherits it from here.
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