121
121.5 megahertz is the international aeronautical emergency frequency.
Where 121 fits
Why 121 is special
Satellite monitoring of it ended in 2009, so a beacon transmitting only on this frequency now depends on an aircraft happening to hear it.
Factors & digit properties
- Divisors (3)
- 1, 11, 121
- Prime factorisation
- 11^2
- Digit sum
- 4
- Digital root
- 4
Representations
- Decimal
- 121
- Scientific notation
- 1.210 × 10^2
Roman numerals
C X X I
Binary
1 1 1 1 0 0 1
1 × 64 + 1 × 32 + 1 × 16 + 1 × 8 + 0 × 4 + 0 × 2 + 1 × 1 = 121
Octal
1 7 1
1 × 64 + 7 × 8 + 1 × 1 = 121
Hexadecimal
7 9
7 × 16 + 9 × 1 = 121
Read as powers of two: 64 + 32 + 16 + 8 + 1 = 121.
In historical numeral systems
Egyptian
1 coil of rope, 2 hobbles, 1 stroke
1 × 100 + 2 × 10 + 1 × 1 = 121. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
2 vertical wedges (sixties place), 1 vertical wedge (units place)
2 × 60 + 1 = 121. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
rho, then kappa, then alpha, marked as a numeral
rho-kappa-alpha + keraia
rho + 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
qof, then kaf, then alef
qof-kaf-alef
qof (100) + kaf (20) + alef (1) = 121, written in descending order.
Maya
1 bar + 1 dot (upper, twenties place) over 1 dot (lower, units place)
6 × 20 + 1 = 121. 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
- Palindromic number
- Deficient number
- Powerful number
- Semiprime
Related numbers & connections
121 appears in Numbers in radio and communications, the topic covering numbers that carry a documented radio or communications fact.
Satellites stopped listening to the analogue distress frequency in 2009 in favour of the digital one.
View 406 →Numbers Around Us
- appears inRadio and communications
why?
Strength 0.50, confidence "high".
Other connections
- contrasts with406
why?
Strength 0.50, confidence "high".
Technology & computing
121.5 MHz is the international aeronautical emergency frequency. Cospas-Sarsat satellite processing of 121.5 MHz beacons ceased in February 2009 in favour of 406 MHz.
DocumentedTechnology & computingGeography: International — ICAOPeriod: 1950s–There are 121 point-five megahertz, the international aeronautical emergency frequency (worldwide, ICAO Annex 10). Satellite monitoring of this frequency ended in 2009, so a beacon transmitting only on it depends on an aircraft hearing it.
DocumentedTechnology & computingGeography: worldwide, ICAO Annex 10
Common questions
Is 121 a prime number?
No, 121 is not prime.
What are the factors of 121?
The factors of 121 are 1, 11, 121.
What is 121 in Roman numerals?
121 in Roman numerals is CXXI.
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