Number reference
13 explained.
13: thirteen, prime, fibonacci number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- thirteen
- Classification
- prime
- Factors
- 1, 13
- Prime factorization
- 13
- Square root
- 3.605551
- Cube root
- 2.351335
- Binary
- 1101
- Octal
- 15
- Hexadecimal
- D
- Roman numeral
- XIII
- Digit sum
- 4
- Digital root
- 4
What makes 13 notable
- Prime. 13 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. 13 appears in the Fibonacci sequence, where each term is the sum of the two before it.
13 in everyday use
Thirteen is widely treated as unlucky in Western superstition, to the point that many buildings label the floor above twelve as fourteen and some airlines skip row 13.
Nearby numbers
How to read this page
Because 13 is prime, the factor list stops at 1 and 13 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 13 in its prime factorisation inherits it from here.
Percentage calculator · Fraction calculator · Browse Number Explorer