Systems Of Units

The most commonly used unit systems include the International SI system of units, the US system of units, and the Imperial (mostly UK) system of units. Some of the most important units are used to measure weight, length, area, volume, temperature, pressure, etc.

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Most Popular Weight Units

SI weight unit for weight is a kilogram (or often just 'kilo'), while non-SI units are Imperial Ton, Tonne, 1 US Ton, Stone, Pound, and Ounce.

Other units may be used as well, but they are not used that often, like grain, drachm, slug, etc.

Non-SI Weight Units
SI Weight Units
1 Imperial Ton = 2240 Pounds 1016.05 kg
 1 Tonne = 2204.62 Pounds 1000 kg
1 US Ton = 2000 Pounds 907.185 kg
1 Quarter = 28 Pounds 12.70058 kg
1 Stone = 14 Pounds 6.35029 kg
1 Pound 0.45359237 kg
1 Ounce =  1/16 Pound = 0.0625 Pounds 0.0283495 kg = 28.3495g

Most Popular Units of Length, Area, and Volume

The most popular non-SI units for length are Nautical Mile, Mile, Yard, Foot, and Inch. Again, there are other units as well, but these are the most popular and still in frequent use.

Non-SI Lenght Units SI Length Units
1 Nautical Mile = 2025.37 Yards 1852 m
1 Mile = 1760 Yards 1609.34 m
1 Yard = 3 feet = 36 inches 0.9144 m = 91.44 cm
1 Foot = 0.3333 Yard = 12 Inches 0.3048 m = 30.48 cm
1 Inch 0.0254 m = 2.54 cm

The units for volume are often derived from the units of length, but not necessarily, for example: 

Non-SI Volume Units SI Volume Units
1 Cubic Yard = 27 Cubic Feet 0.764554857984 m3 = 764.554857984 liters
1 Cubic Foot = 1728 Cubic Inches = 957.506 US Fluid Ounces 0.0283168 m3 =  28.3168 liters
Dry Quart (US) = 2 Dry Pints (US) = 67.2 Cubic Inches 1101.22 milliliters = 1.10122 liters
Dry Pint (US) = 33.6 Cubic Inches 550.6 milliliters = 0.5506 liters
1 Cubic Inch =  0.554113 US Fluid Ounces  0.0163871 liters = 16.3871 milliliters

Note: 1 Cubic Meter (m3) = 1000 liters = 1000 dm3

Also, sometimes, the Imperial (British) system uses one unit (quart, pint) for liquid and dry measures, while the US system uses two units (dry quart, liquid quart, dry pint, liquid pint).

Probably the largest source of confusion among the units is the conversion between the US measurement system and the Imperial (UK) measurement system:

US Units (Volume)
Imperial Units (Volume)
1 US Liquid Gallon = 128 US Fluid Ounces = 3.78541 liters 1 Imp Liquid Gallon = 160 Imp Fluid Ounces =  4.54609 liters
1 US Liquid Gallon =  0.832674 Imp Liquid Gallon 1 Imp Liquid Gallon =  1.20095 US Liquid Gallon
1 US Liquid Quart = 32 US Fluid Ounces = 0.946353 liters 1 Imp Liquid Quart =  40 Imp Fluid Ounces = 1.13652 liters
1 US Liquid Quart =  0.832674 Imp Liquid Quart 1 Imp Liquid Quart =  1.20095 US Liquid Quart
1 US Liquid Pint = 16 US Fluid Ounces = 0.473176 liters 1 Imp Liquid Pint =  20 Imp Fluid Ounces = 0.568261 liters
1 US Liquid Pint =  0.832674 Imp Liquid Pint 1 Imp Liquid Pint = 1.20095 US Liquid Pint
1 US Customary Cup = 8 US Fluid Ounces = 0.2365882365 liters 1 Imp Cup = 10 Imp Fluid Ounces = 0.284131 liters
1 US Customary Cup =  0.832673 Imp Cup 1 Imp Cup =  1.200951 US Customary Cups
1 US Legal Cup = 240 milliliters 1 Imp Cup = 1.183879 US Legal Cups
1 US Fluid Ounce = 29.5735 milliliters 1 Imp Fluid Ounce = 28.4131 milliliters
1 US Fluid Ounce = 1.04084 Imp Fluid Ounces 1 Imp Fluid Ounce = 0.96076 US Fluid Ounces
1 US Tablespoon = 1/2 US Fluid Ounces = 14.7868 milliliters 1 Imp Tablespoon = 0.625 Imp Fluid Ounces = 17.7582 milliliters
1 US Tablespoon = 0.832674 Imp Tablespoon 1 Imp Tablespoon =  1.20095 US Tablespoon
1 US Teaspoon = 1/6 US Fluid Ounces = 4.92892 milliliters 1 Imp Teaspoon = 0.208333 Imp Fluid Ounces = 5.91939 milliliters
1 US Teaspoon = 0.832674 Imp Teaspoon 1 Imp Teaspoon = 1.20095 US Teaspoon

 International System (SI) of Units Official Prefixes

To write extra small and extra large values more easily, the International System (SI) of Units uses prefixes that range from yocto (y, 10-24) to yotta (Y, 1024).

The following chart lists the International System of Units (SI) official prefixes:

Prefix Base 10 Decimal English Word
Name Symbol Short Scale Long Scale
quetta Q 1030 1 000 000 000 000 000 000 000 000 000 000 nonillion quintillion
ronna R 1027 1 000 000 000 000 000 000 000 000 000 octillion quadrilliard
yotta Y 1024 1 000 000 000 000 000 000 000 000 septillion quadrillion
zetta Z 1021 1 000 000 000 000 000 000 000 sextillion trilliard
exa E 1018 1 000 000 000 000 000 000 quintillion trillion
peta P 1015 1 000 000 000 000 000 quadrillion billiard
tera T 1012 1 000 000 000 000 trillion billion
giga G 109 1 000 000 000 billion milliard
mega M 106 1 000 000 million
kilo k 103 1 000 thousand
hecto h 102 100 hundred
deca da 101 10 ten
- - 100 1 one
deci d 10-1 0.1 tenth
centi c 10-2 0.01 hundredth
milli m 10-3 0.001 thousandth
micro μ 10-6 0.000 001 millionth
nano n 10-9 0.000 000 001 billionth milliardth
pico p 10-12 0.000 000 000 001 trillionth billionth
femto f 10-15 0.000 000 000 000 001 quadrillionth billiardth
atto a 10-18 0.000 000 000 000 000 001 quintillionth trillionth
zepto z 10-21 0.000 000 000 000 000 000 001 sextillionth trilliardth
yocto y 10-24 0.000 000 000 000 000 000 000 001 septillionth quadrillionth
ronto r 10-27 0.000 000 000 000 000 000 000 000 001 octillionth quadrilliardth
quecto q 10-30 0.000 000 000 000 000 000 000 000 000 001 nonillionth quintillionth

Note: the 'Short Scale' is in use, for example, in the United States, while the 'Long Scale' is in use, for example, in Europe - this may be the largest source of mistakes and errors in everyday speech and conversation.


Personally, the use of the SI system simplifies math greatly, but tradition is tradition. Nonetheless, just in case, it is good to know relations between various units and how to convert from to another and vice versa.