221
221B Baker Street is Sherlock Holmes's address.
- Odd
- Composite
Where 221 fits
Why 221 is special
When Conan Doyle wrote the stories Baker Street numbers did not reach 221, so the address was invented. The street was later extended, and the building that acquired the number received so much mail addressed to Holmes that a bank employed someone to answer it.
Factors & digit properties
- Divisors (4)
- 1, 13, 17, 221
- Prime factorisation
- 13 × 17
- Digit sum
- 5
- Digital root
- 5
Representations
- Decimal
- 221
- Scientific notation
- 2.210 × 10^2
Roman numerals
C C X X I
Binary
1 1 0 1 1 1 0 1
1 × 128 + 1 × 64 + 0 × 32 + 1 × 16 + 1 × 8 + 1 × 4 + 0 × 2 + 1 × 1 = 221
Octal
3 3 5
3 × 64 + 3 × 8 + 5 × 1 = 221
Hexadecimal
d d
d × 16 + d × 1 = 221
Read as powers of two: 128 + 64 + 16 + 8 + 4 + 1 = 221.
In historical numeral systems
Egyptian
2 coils of rope, 2 hobbles, 1 stroke
2 × 100 + 2 × 10 + 1 × 1 = 221. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
3 vertical wedges (sixties place), 4 corner wedges + 1 vertical wedge (units place)
3 × 60 + 41 = 221. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
sigma, then kappa, then alpha, marked as a numeral
sigma-kappa-alpha + keraia
sigma + kappa + alpha written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
resh, then kaf, then alef
resh-kaf-alef
resh (200) + kaf (20) + alef (1) = 221, written in descending order.
Maya
2 bars + 1 dot (upper, twenties place) over 1 dot (lower, units place)
11 × 20 + 1 = 221. 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
- Semiprime
Media & popular culture
Baker Street numbers did not reach 221 when Conan Doyle was writing, so Holmes's address was safely fictional. The street was later extended, and the building that received the number attracted so much mail addressed to Holmes that a bank employed a secretary to answer it.
DocumentedMedia & popular cultureGeography: London, United Kingdom
Common questions
Is 221 a prime number?
No, 221 is not prime.
What are the factors of 221?
The factors of 221 are 1, 13, 17, 221.
What is 221 in Roman numerals?
221 in Roman numerals is CCXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 1 reviewer-confirmed record, sourced and reviewed before publication.
Last reviewed: 15 July 2026