55
55 is the atomic number of caesium — the element whose radiation frequency defines the SI second — and, from 1974 to 1995, the US National Maximum Speed Limit in miles per hour.
Where 55 fits
Why 55 is special
55 links precise atomic-clock science (caesium) to a specific, now-repealed piece of 1970s US energy policy.
Same number, different meanings
How 55 is understood across different contexts — never implying one meaning because of another.
- Mathematics
55 links precise atomic-clock science (caesium) to a specific, now-repealed piece of 1970s US energy policy.
- Science
55 is the atomic number of caesium; a fixed number of radiation oscillations from a caesium-133 atom is the modern scientific definition of the SI second, the basis of every atomic clock.
- History
55 mph was the United States' National Maximum Speed Limit from 1974 to 1995, introduced for fuel conservation during the 1973 oil crisis before being repealed and returned to individual states.
55 is part of
Factors & digit properties
- Divisors (4)
- 1, 5, 11, 55
- Prime factorisation
- 5 × 11
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 55
- Binary
- 110111
- Octal
- 67
- Hexadecimal
- 37
- Roman numeral
- LV
- Scientific notation
- 5.500 × 10^1
Other mathematical patterns
- Triangular number
- Palindromic number
- Deficient number
- Repdigit
- Kaprekar number
- Semiprime
Related numbers & connections
11 is a factor of 55 (11 × 5 = 55).
View 11 →Mathematically related
- has factor11
why?
Strength 0.50, confidence "high".
Science
55 is the atomic number of caesium; a fixed number of radiation oscillations from a caesium-133 atom is the modern scientific definition of the SI second, the basis of every atomic clock.
DocumentedScience
History
55 mph was the United States' National Maximum Speed Limit from 1974 to 1995, introduced for fuel conservation during the 1973 oil crisis before being repealed and returned to individual states.
DocumentedHistoryGeography: United StatesPeriod: 1974-1995
Common questions
Is 55 a prime number?
No, 55 is not prime.
What are the factors of 55?
The factors of 55 are 1, 5, 11, 55.
What is 55 in Roman numerals?
55 in Roman numerals is LV.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 2 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026