55
55 is the tenth Fibonacci number and the tenth triangular number at once, a rare double. It was the US national speed limit from 1974 to 1995, and it is the atomic number of caesium, whose radiation frequency defines the SI second.
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55 links precise atomic-clock science (caesium) to a specific, now-repealed piece of 1970s US energy policy.
55 connects to:
- 11 — has factor
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55 links precise atomic-clock science (caesium) to a specific, now-repealed piece of 1970s US energy policy.
- 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Maximum Speed Limit from 1974 to 1995, introduced for fuel conservation during the 1973 oil crisis before being repealed and returned to individual states.
- Law & regulation
The United States imposed a national maximum speed limit of 55 mph in 1974, as a fuel-saving response to the oil crisis rather than a road-safety measure. It was widely disliked and widely ignored, relaxed to 65 mph on rural interstates in 1987, and repealed entirely in 1995, returning the decision to individual states.
- 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.
Factors & digit properties
- Divisors (4)
- 1, 5, 11, 55
- Prime factorisation
- 5 × 11
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 55
- Scientific notation
- 5.500 × 10^1
Roman numerals
L V
Binary
1 1 0 1 1 1
1 × 32 + 1 × 16 + 0 × 8 + 1 × 4 + 1 × 2 + 1 × 1 = 55
Octal
6 7
6 × 8 + 7 × 1 = 55
Hexadecimal
3 7
3 × 16 + 7 × 1 = 55
Read as powers of two: 32 + 16 + 4 + 2 + 1 = 55.
In historical numeral systems
Egyptian
5 hobbles, 5 strokes
5 × 10 + 5 × 1 = 55. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
5 corner wedges + 5 vertical wedges
55 fits in a single sexagesimal place: 5 corner wedges + 5 vertical wedges.
Greek (Ionic)
nu, then epsilon, marked as a numeral
nu-epsilon + keraia
nu + epsilon 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 he
nun-he
nun (50) + he (5) = 55, written in descending order.
Chinese numerals
wǔ shí wǔ
wǔ shí wǔ
五 before 十 multiplies it: 5 tens. 五 after it adds 5. The written form mirrors the arithmetic — "wǔ shí wǔ". The financial (anti-fraud) form is 伍拾伍.
Maya
2 dots (upper, twenties place) over 3 bars (lower, units place)
2 × 20 + 15 = 55. 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
- 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".
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
Law & regulation
The United States imposed a national maximum speed limit of 55 mph in 1974, as a fuel-saving response to the oil crisis rather than a road-safety measure. It was widely disliked and widely ignored, relaxed to 65 mph on rural interstates in 1987, and repealed entirely in 1995, returning the decision to individual states.
DocumentedLaw & regulationGeography: United StatesPeriod: 1974–1995
Mathematics
55 is both the 10th Fibonacci number and the 10th triangular number — the sum of 1 through 10. Very few numbers appear in both sequences at all, and fewer still occupy the same position in each.
DocumentedMathematics
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
Common questions
Is 55 both a Fibonacci number and a triangular number?
Yes, and it is the tenth of each — the tenth Fibonacci number and the sum of 1 to 10. The overlap is rare: 1, 3, 21 and 55 are the only numbers known to be both, and no others have ever been found.
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: 4 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026