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Imperial units are the old British system now used mostly in the USA. Yards, miles, feet, inches. Pounds, ounces, gallons, etc.

Metric units are the SI or MKS system, Meters, liters, kilograms and all the derivatives with prefixes.

Imperial units were established by the Imperial Weights and Measures Act 1824, as amended.  The primary standards were lost in the fire of 1834, but new standards restored and legalised by act of 1855.

Until 1959, the Imperial standards were defined by separate standards, and various comparisons were legalised for conversion factors in 1867 (Kater + Broch), 1879 (Clarke, Broch), 1897 (Beniot), 1930 (Seers and Jolly) and 1963 (International agreement).

The USC measures are the pre-imperial units used in america, variously defined in 1800, 1867, 1895 (Mendenhall), and 1959 (International agreement).

The older conversion factors continue as definitions used in geodetic spheres, but the use of the GPS, based on the 1984 Ellipsoid have rendered these largely obsolete.

Metric units:

• most based on SI system
• the greater unit is powered ten times bigger than smaller one (for example: L is 10^3 times bigger than mL)
• have prefixes (milli, centi, deci, kilo, mega, etc…)
• is used widely on the world

Imperial units:

• not based or have any relation to SI system
• the greater unit is not a powered ten times bigger than the smaller one (example: A feet is twelve times bigger than an inch)
• have no prefixes
• mostly used in some country in Europe an America

Metric units all fit together: 1 litre of water weighs 1kg. SI units: 1 newton is the force required to give a mass of 1 kg an acceleration of 1 m/s^2: 1 joule is the work done when a force of 1 newton moves a mass of 1kg a distance of 1m in the direction on the force… and so on.

Imperial is traditional and ad hoc (I believe a length of one inch is equal to the length of three barleycorns) whereas metric is thoughtful and pragmatic (1 metre = distance travelled by light in 1/299 792 458 seconds.