13
13 is a prime number widely — though not universally — considered unlucky in Western tradition.
Where 13 fits
Why 13 is special
Its ordinary mathematical profile (prime, immediately after the highly composite 12) is overshadowed by triskaidekaphobia, a documented cultural phenomenon that shapes real building design.
See 13 visually
Same number, different meanings
How 13 is understood across different contexts — never implying one meaning because of another.
- Mathematics
Its ordinary mathematical profile (prime, immediately after the highly composite 12) is overshadowed by triskaidekaphobia, a documented cultural phenomenon that shapes real building design.
- Science
13 is the atomic number of aluminium, the most abundant metal in Earth's crust.
- Culture
Triskaidekaphobia — fear or avoidance of the number 13 — is a well-documented Western cultural phenomenon; many hotels, hospitals and apartment buildings omit a labelled 13th floor, and some airlines omit row 13.
13 in Numbers Around Us
13 is part of
Factors & digit properties
- Divisors (2)
- 1, 13
- Prime factorisation
- 13
- Digit sum
- 4
- Digital root
- 4
Representations
- Decimal
- 13
- Binary
- 1101
- Octal
- 15
- Hexadecimal
- d
- Roman numeral
- XIII
- Scientific notation
- 1.300 × 10^1
Other mathematical patterns
- Deficient number
- Happy number
- Lucky number (mathematical sieve)
- Twin prime
Related numbers & connections
13 is a factor of 26 (13 × 2 = 26).
View 26 →8 and 13 are consecutive Fibonacci numbers.
View 8 →13 is a member of the Fibonacci sequence.
Mathematically related
Nearby in a sequence
Shares a distinctive class
- belongs toFibonacci sequence
why?
Strength 0.50, confidence "high".
Scientific association
- has scientific association withAluminium (chemical element)
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inPhyllotaxis petal patterns
why?
Strength 0.50, confidence "high".
- appears inPeriodical cicada emergence cycles
why?
Strength 0.50, confidence "high".
Culturally related
- is considered unlucky inWestern/Christian-influenced contexts
why?
Strength 0.50, confidence "high".
Other connections
- is associated withTriskaidekaphobia
why?
Strength 0.50, confidence "high".
- contrasts with7belief/tradition
why?
Strength 0.50, confidence "medium".
Science
13 is the atomic number of aluminium, the most abundant metal in Earth's crust.
DocumentedScience13 is the atomic number of aluminium, the most abundant metal in Earth's crust.
DocumentedScience
Culture
Triskaidekaphobia — fear or avoidance of the number 13 — is a well-documented Western cultural phenomenon; many hotels, hospitals and apartment buildings omit a labelled 13th floor, and some airlines omit row 13.
Widely documentedCultureCulture: Western
Common questions
Is 13 a Fibonacci number?
Yes — 13 is the sum of the two Fibonacci numbers before it, 5 and 8 (5 + 8 = 13), making it the 7th number in the sequence.
What element has atomic number 13?
Aluminium — 13 is its atomic number, meaning every aluminium atom has 13 protons.
Is 13 unlucky everywhere?
No. The association is well-documented in Western cultures (floor and row omission, for example), but it isn't universal — Italy has a long-documented tradition treating 13 as a lucky number instead.
Does 13 have a twin prime?
Yes — 11 and 13 are twin primes, a pair of primes exactly two apart. (13 and 17, its next prime neighbour, are four apart, so that pair doesn't count as twins.)
Is 13 a prime number?
Yes, 13 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 13?
The factors of 13 are 1, 13.
What is 13 in Roman numerals?
13 in Roman numerals is XIII.
Is 13 a lucky number?
13 is considered unlucky in Western/Christian-influenced contexts — a documented cultural association, not a mathematical property.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 3 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026