FFF system

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The furlong–firkin–fortnight (FFF) system is a humorous system of units based on unusual or impractical measurements. The length unit of the system is the furlong, the mass unit is the mass of a firkin of water, and the time unit is the fortnight.[1][2] Like the SI or metre–kilogram–second systems, there are derived units for velocity, volume, mass and weight, etc. It is sometimes referred as FFFF system where the fourth 'F' is degrees Fahrenheit for temperature.

While the FFF system is not used in practice it has been used as an example in discussions of the relative merits of different systems of units.[1][3] Some of the FFF units, notably the microfortnight, have been used jokingly in computer science. Besides having the meaning "any obscure unit",[4] the derived unit furlongs per fortnight has also served frequently in classroom examples of unit conversion and dimensional analysis.[5][6]

Base units and definitions[edit]

Unit Abbreviation Dimension SI unit Imperial unit
furlong fur length 201.168 m 220 yards
firkin fir mass 40.8233133 kg 90 lb[a]
fortnight ftn time 1,209,600 s 14 days

Notable multiples and derived units[edit]

Microfortnight and other decimal prefixes[edit]

One microfortnight is equal to 1.2096 seconds.[2] This has become a joke in computer science because in the VMS operating system, the TIMEPROMPTWAIT variable, which holds the time the system will wait for an operator to set the correct date and time at boot if it realizes that the current value is invalid, is set in microfortnights. This is because the computer uses a loop instead of the internal clock, which has not been activated yet to run the timer.[7] The documentation notes that "[t]he time unit of micro-fortnights is approximated as seconds in the implementation".[8]

The Jargon File reports that the millifortnight (about 20 minutes) and nanofortnight have been occasionally used.[7]

Furlongs per fortnight[edit]

One furlong per fortnight is a speed which would be barely noticeable to the naked eye. It converts to:

  • 1.663×10−4 m/s, (i.e. 0.1663 mm/s),
  • roughly 1 cm/min (to within 1 part in 400),[b]
  • 5.987×10−4 km/h,
  • roughly 38 in/min,
  • 3.720×10−4 mph.

Speed of light[edit]

The speed of light is 1.8026×1012 furlongs per fortnight (1.8026 megafurlongs per microfortnight). By mass–energy equivalence, 1 firkin is equal to 3.24936676×1024 firkin·furlong2/fortnight2 (≈ 3.478×1015 BTU, or 3.669×1018 joules).


In the FFF system, heat transfer coefficients are conventionally reported as BTU per foot-fathom per degree Fahrenheit per fortnight.[c] Thermal conductivity has units of BTU per fortnight per furlong per degree Fahrenheit.

Like the more common furlongs per fortnight,[4] firkins per fortnight have been used with the meaning "any obscure unit".[10]

See also[edit]


  1. ^ The firkin of the FFF System is defined as the mass of an imperial firkin (9 imp gal) of water. The imperial gallon was originally defined as the volume of 10 lb of distilled water (weighed according to specific conditions). From this definition a density of 10 lb/imp gal is derived, giving the firkin of water a mass of 90 lb.
  2. ^ Indeed, if the inch were defined as 2.5454... cm, it would be 1 cm/min[9]
  3. ^ The foot-fathom is a unit of area; 1 foot-fathom is equal to 6 square feet.


  1. ^ a b Kelly-Bootle, Stan (March 2007), "As Big as a Barn?", ACM Queue, pp. 62–64
  2. ^ a b Slade, Robert (2006). Dictionary of information security. Syngress. p. 122. ISBN 1-59749-115-2.
  3. ^ Neff, John D. (June 1983). "Imbedding the Metric". The Two-Year College Mathematics Journal. 14 (3): 197–202. doi:10.2307/3027087. JSTOR 3027087.
  4. ^ a b Ganssle, Jack G. (2008). The art of designing embedded systems (2nd ed.). Newnes. p. 50. ISBN 978-0-7506-8644-0.
  5. ^ Giambattista, Alan; Richardson, Betty McCarthy & Richardson, Robert C. (2004). College Physics. Boston: McGraw Hill. p. 20. ISBN 0-07-052407-6.
  6. ^ Stephan, Elizabeth A.; Park, William J.; Sill, Benjamin L.; Bowman, David R. & Ohland, Matthew W. (2010). Thinking Like an Engineer: An Active Learning Approach. Prentice Hall. p. 259. ISBN 978-0-13-606442-8.
  7. ^ a b "microfortnight". Retrieved 2007-07-06.
  8. ^ "HP OpenVMS System Management Utilities Reference Manual". Archived from the original on 2018-08-24.
  9. ^ "FAQ for newsgroup UK.rec.sheds, version 2&3/7th". 2000. Archived from the original (TXT) on 2006-03-06. Retrieved 2006-03-10.
  10. ^ Page-Jones, Meilir & Constantine, Larry L. (2000). Fundamentals of object-oriented design in UML. Addison–Wesley. p. 235. ISBN 0-201-69946-X.