51
Area 51 is a US Air Force facility in Nevada, named from its map grid designation and used since 1955 to test classified aircraft. 51 is also the sixth pentagonal number, and the atomic number of antimony.
- Odd
- Composite
Where 51 fits
Why 51 is special
51's cultural fame comes entirely from Area 51's decades of conspiracy-theory notoriety, not from any mathematical property (it factors quietly as 3 × 17).
Same number, different meanings
How 51 is understood across different contexts — never implying one meaning because of another.
- Mathematics
51's cultural fame comes entirely from Area 51's decades of conspiracy-theory notoriety, not from any mathematical property (it factors quietly as 3 × 17).
- History
The Nevada site was established in 1955 to flight-test the U-2 reconnaissance aircraft, and later the A-12 and SR-71. The CIA acknowledged its existence and that programme in documents declassified in 2013 — which also supplied a mundane explanation for decades of sightings, since aircraft of unfamiliar shape were being flown at altitudes no civil aircraft reached.
- Culture
'The 51st state' is a standing political shorthand in the United States for a territory that might be admitted next — most often Puerto Rico or the District of Columbia — and is used elsewhere, sometimes pejoratively, for a country seen as closely aligned with US interests.
- Science
51 is the atomic number of antimony, used mainly to harden lead alloys and, as antimony trioxide, as one of the most widely used flame retardants in plastics and textiles.
Factors & digit properties
- Divisors (4)
- 1, 3, 17, 51
- Prime factorisation
- 3 × 17
- Digit sum
- 6
- Digital root
- 6
Representations
- Decimal
- 51
- Scientific notation
- 5.100 × 10^1
Roman numerals
L I
Binary
1 1 0 0 1 1
1 × 32 + 1 × 16 + 0 × 8 + 0 × 4 + 1 × 2 + 1 × 1 = 51
Octal
6 3
6 × 8 + 3 × 1 = 51
Hexadecimal
3 3
3 × 16 + 3 × 1 = 51
Read as powers of two: 32 + 16 + 2 + 1 = 51.
In historical numeral systems
Egyptian
5 hobbles, 1 stroke
5 × 10 + 1 × 1 = 51. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
5 corner wedges + 1 vertical wedge
51 fits in a single sexagesimal place: 5 corner wedges + 1 vertical wedge.
Greek (Ionic)
nu, then alpha, marked as a numeral
nu-alpha + keraia
nu + alpha written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
nun, then alef
nun-alef
nun (50) + alef (1) = 51, written in descending order.
Chinese numerals
wǔ shí yī
wǔ shí yī
五 before 十 multiplies it: 5 tens. 一 after it adds 1. The written form mirrors the arithmetic — "wǔ shí yī". The financial (anti-fraud) form is 伍拾壹.
Maya
2 dots (upper, twenties place) over 2 bars + 1 dot (lower, units place)
2 × 20 + 11 = 51. Places stack vertically with the smallest at the bottom, so the upper group is worth twenty times the lower.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Deficient number
- Pentagonal number
- Semiprime
- Lucky number (mathematical sieve)
Related numbers & connections
17 is a factor of 51 (17 × 3 = 51).
View 17 →Ecuador and Peru share a border that was disputed into the 1990s. Ecuador was the first country to write rights of nature into its constitution.
View 593 →Fifty percent is the reference point for a majority, which is why marginal cases cluster immediately either side of it.
View 50 →History
The Nevada site was established in 1955 to flight-test the U-2 reconnaissance aircraft, and later the A-12 and SR-71. The CIA acknowledged its existence and that programme in documents declassified in 2013 — which also supplied a mundane explanation for decades of sightings, since aircraft of unfamiliar shape were being flown at altitudes no civil aircraft reached.
DocumentedHistoryGeography: Nevada, United StatesPeriod: 1955 onwards
Culture
'The 51st state' is a standing political shorthand in the United States for a territory that might be admitted next — most often Puerto Rico or the District of Columbia — and is used elsewhere, sometimes pejoratively, for a country seen as closely aligned with US interests.
DocumentedCultureGeography: United States and English-speaking political commentary'Area 51' is a secretive United States Air Force facility in Nevada whose name — from its map grid designation — has made it a long-running subject of public fascination and conspiracy theories.
Widely documentedCultureGeography: United States
Mathematics
51 is the sixth pentagonal number — the count of dots in a pentagon built up in nested rings, after 1, 5, 12, 22 and 35. It is also 3 times 17, which surprises people who expect a number ending in 1 to be prime; the final digit never settles the question.
DocumentedMathematics
Science
51 is the atomic number of antimony, used mainly to harden lead alloys and, as antimony trioxide, as one of the most widely used flame retardants in plastics and textiles.
DocumentedScience
Common questions
What is Area 51?
A United States Air Force facility in Nevada, used since 1955 for flight-testing classified aircraft including the U-2 and the SR-71. The government did not formally acknowledge its existence until 2013. The secrecy, the unexplained aircraft and the location did the rest: the extraterrestrial associations attach to the base rather than to the number, which is simply a map grid designation.
What actually happens at Area 51?
Classified aircraft testing. The site was set up in 1955 for the U-2 spy plane and later used for the A-12 and SR-71, and the CIA formally acknowledged that in documents declassified in 2013. That also explains a good deal of the sighting history: aircraft of unfamiliar shape were being flown at altitudes no civil aircraft reached, over a facility whose existence the government would not confirm.
Is 51 a prime number?
No, 51 is not prime.
What are the factors of 51?
The factors of 51 are 1, 3, 17, 51.
What is 51 in Roman numerals?
51 in Roman numerals is LI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 5 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026