The FPS (foot–pound–second) system is built on three fundamental units:
- foot for length
- (avoirdupois) pound for mass
- second for time.
The foot is defined as being equivalent exactly to 0.3048 m.
One foot represents a length of twelve inches, and three feet equal one yard.
The international avoirdupois pound is defined as exactly 0.453 592 37 kg.
One pound is divided into sixteen ounces.
The second is defined as the time that elapses during
There are sixty seconds in a minute, sixty minutes in an hour, and twenty-four hours in a day.
The corresponding SI units are the metre, kilogram, and second.
The metre is defined as the distance traveled by light in a vacuum
The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), a platinum-iridium cylinder that is kept in a high-security vault outside Paris.
1 kg = 1000 g, and 1 g = 1000 mg
The definition of the second is the same in both the FPS system and the SI.
The Big Differences
(a) FPs is not coherent
In a coherent system of units, all derived units are derived by simple multiplication or division without only "1" as a proportionality factor.
For example, in the SI, the standard unit of volume is #"1 m"^3".
In the FPS system, the gallon is not equal to a cubic yard or the cube of any length unit.
(b) SI fractions and multiples of units are expressed as powers of 10.
1 km = 1000 m; 1 m = 1000 mm, etc.
In the FPs system, 1 mi = 1760 yd; 1 yd = 3 ft; 1 ft = 12 in.