Number reference
55 explained.
55: fifty-five, composite, triangular number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- fifty-five
- Classification
- composite
- Factors
- 1, 5, 11, 55
- Prime factorization
- 5 × 11
- Square root
- 7.416198
- Cube root
- 3.802952
- Binary
- 110111
- Octal
- 67
- Hexadecimal
- 37
- Roman numeral
- LV
- Digit sum
- 10
- Digital root
- 1
What makes 55 notable
- Triangular number. 55 is the 10th triangular number: it is the sum 1 + 2 + … + 10, the count of items in a triangle with 10 rows.
- Fibonacci number. 55 appears in the Fibonacci sequence, where each term is the sum of the two before it.
- Repdigit. 55 is written with a single repeated digit, so it is divisible by the repunit of the same length.
55 in everyday use
Fifty-five miles per hour was the national speed limit in the United States from 1974 to 1995, imposed to save fuel after the oil crisis. It is the atomic number of caesium, whose atoms define the second in atomic clocks. Fifty-five delegates attended the Constitutional Convention in Philadelphia in 1787, though only 39 of them signed the finished document.
Fifty-five is the tenth Fibonacci number, the sum of 21 and 34, and also the tenth triangular number, 1 + 2 + … + 10. It is the largest Fibonacci number that is also triangular. It is a square pyramidal number too, 1 + 4 + 9 + 16 + 25, and a repdigit, 5 × 11. In the Fibonacci sequence it is the largest term that is a two-digit number.
Nearby numbers
How to read this page
The factor list shows every way 55 splits into equal whole-number groups — 4 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 55 for contexts that do not use decimal.
Percentage calculator · Fraction calculator · Browse Number Explorer