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
- Binary
- 11011101
- Octal
- 335
- Hexadecimal
- dd
- Roman numeral
- CCXXI
- Scientific notation
- 2.210 × 10^2
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: 3 August 2026