British Imperial System
A principal system of units.
English units forms the basic for the Imperial system formerly used in the Commonwealth countries and the Customary system used in the US.
The basic difference between dimensional systems are not the units employed but the primary dimensions.
 The SIsystem use mass, length and time as primary quantities (mechanics). Other units quantities are secondary. The SIsystem is a MLT (masslengthtime) system
 The English system use force, mass, length and time as primary quantities. Other units are secondary. The British Imperial System is a FMLT (forcemasslengthtime) system
Note that the use of four, three, two or even one(!) primary quantities is a human choice and not necessary basic nature.
Imperial Primary Quantities
Imperial primary quantities are expressed in the table below:
Primary Quantity  Dimensions  Common Units 

Force  F  poundforce 
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 are expressed in the table below
Primary Quantity  Dimensions  Common Units 

Acceleration  L / T^{2}  foot per second squared 
Area  L^{2}  square foot, square yard acre, square mile 
Density  M / L^{3}  pounds per cubic foot 
Frequency  1/T  cycle per second 
Impulse  F T  poundforce second 
Momentum  ML / T  pound foot per second 
Power  LF / T  foot poundforce per second horsepower 
Pressure  F / L^{2}  poundforce per square inch 
Velocity  L / T  foot per second mile per hour, knot 
Volume  L^{3}  pint, quart, gallon cubic foot, cubic yard 
Work  L F  foot poundforce 
Related Topics

Basics
The SIsystem, unit converters, physical constants, drawing scales and more.
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