Number reference
51 explained.
51: fifty-one, composite; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- fifty-one
- Classification
- composite
- Factors
- 1, 3, 17, 51
- Prime factorization
- 3 × 17
- Square root
- 7.141428
- Cube root
- 3.70843
- Binary
- 110011
- Octal
- 63
- Hexadecimal
- 33
- Roman numeral
- LI
- Digit sum
- 6
- Digital root
- 6
What makes 51 notable
- Semiprime. 51 = 3 × 17, a product of exactly two primes. That gives it just four divisors, so it splits into equal groups in only two non-trivial ways.
51 in everyday use
Area 51, the US Air Force test site in Nevada, has made this number a shorthand for secrecy since the 1950s. Fifty-one is the atomic number of antimony, an element used in lead alloys and flame retardants. The phrase "51st state" is used for any territory or country imagined joining the United States. Fifty-one percent is the smallest whole-number majority, and 51 cards make a deck that is missing one.
Fifty-one looks prime but is not: it is 3 × 17, one of the classic traps in mental arithmetic alongside 57 and 91. It is a pentagonal number, following 1, 5, 12, 22, and 35. It is also a Motzkin number, following 1, 2, 4, 9, and 21, which count ways of drawing non-crossing chords between points on a circle. In binary it is 110011, two pairs of ones.
Nearby numbers
How to read this page
The factor list shows every way 51 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 51 for contexts that do not use decimal.
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