FFF system

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. Like the SI or metre–kilogram–second systems, there are derived units for velocity, volume, mass and weight, etc. It is sometimes referred as the 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. Some of the FFF units, notably the microfortnight, have been used jokingly in computer science. Besides having the meaning "any obscure unit", the derived unit furlongs per fortnight has also served frequently in classroom examples of unit conversion and dimensional analysis.

Microfortnight and other decimal prefixes
One microfortnight is equal to 1.2096 seconds. 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. The documentation notes that "[t]he time unit of micro-fortnights is approximated as seconds in the implementation".

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

Furlong per fortnight
One furlong per fortnight is a speed that would be barely noticeable to the naked eye. It converts to:
 * 1.663 m/s, (i.e. 0.1663 mm/s),
 * roughly 1 cm/min (to within 1 part in 400),
 * 5.987 km/h,
 * roughly $3/8$ in/min,
 * 3.720 mph,
 * the speed of the tip of a $3 3/4$ inch minute hand.

Speed of light
The speed of light is $1.803$ furlongs per fortnight (1.8026 terafurlongs per fortnight). By mass–energy equivalence, 1 firkin is equal to $3.249 firkin·furlong^{2}⁄fortnight^{2}$ (≈ $3.478 BTU$, or $3.669 joules$).

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

Like the more common furlong per fortnight, a firkin per fortnight can refer to "any obscure unit".