British Imperial System
Principal system of units
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These units form the basis for the Imperial system formerly used in Commonwealth countries and U.S. customary system used in the US.
The basic difference between dimensional systems is not in the units employed but in the primary dimensions.
- The SI-system uses mass, length and time as primary quantities (mechanics), other units quantities are secondary. The SI-system is a MLT (mass-length-time) system.
- The English system uses force, mass, length and time as primary quantities, other units are secondary. The British Imperial System is a FMLT (force-mass-length-time) system
Note that the use of four, three, two or even one(!) primary quantities is a human choice and not basic nature.
Imperial Primary Quantities
| Primary Quantity | Dimensions | Common Units |
| Force | F | pound-force |
| Mass | M | ounce, pound, slug, ton |
| Length | L | inch, foot, yard rod, chain, furlong, mile |
| Time | T | second, minute hour, day |
Imperial Secondary Quantities
Some common imperial secondary quantities:
| Primary Quantity | Dimensions | Common Units |
| Acceleration | L / T2 | foot per second squared |
| Area | L2 | square foot, square yard acre, square mile |
| Density | M / L3 | pounds per cubic foot |
| Frequency | 1/T | cycle per second |
| Impulse | F T | pound-force second |
| Momentum | ML / T | pound foot per second |
| Power | LF / T | foot pound-force per second horsepower |
| Pressure | F / L2 | pound-force per square inch |
| Velocity | L / T | foot per second mile per hour, knot |
| Volume | L3 | pint, quart, gallon cubic foot, cubic yard |
| Work | L F | foot pound-force |
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