British Imperial System

Principal system of units

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.

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

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