Systems of Units
The most widely used system of units and measures around the world is the Systeme International d'Unites (SI), the modern form of the metric system. This originated in France, where in 1790 the French Academy of Science was commissioned to design a new system of units. They decided that
These principles allowed the metric system to evolve
and SI units have become the fundamental basis of scientific measurement
worldwide.
International system of units (SI)
The Système Internationale d'Unitès was adopted in 1960 as the title for an MKSA system based on (meter, kilogram, second, Ampere).
Basic units
Physical quantity 
SI unit 
Unit symbol 
Length 
Meter 
m 
Mass 
Kilogram 
kg 
Time 
Second 
s 
Electric current 
Ampere 
A 
Degree of temperature 
Kelvin 
K 
Amount of substance 
Mole 
mole 
Light (luminous) intensity 
Candela 
cd 
Associated with these basic units are a variety of supplementary derived units which are adopted worldwide.
Derived units
Derived quantity 
SI unit 
Unit symbol 
Area 
Square meter 
m^{2} 
Volume 
Cubic meter 
m^{3} 
Velocity 
Meter per second 
m/s 
Acceleration 
Meter per second squared 
m/s^{2} 
Density 
Kilogram per cubic meter 
kg/m^{3} 
Derived units with special names and symbol
Physical quantity 
SI unit 
Unit symbol 
Force, weight 
newton 
N = kg m/s^{2} 
Work, energy, quantity of heat 
joule 
J =N m 
Power 
watt 
W = J/s 
Pressure, stress 
pascal 
Pa = N/m^{2} 
Electric charge 
coulomb 
C =A s 
Electrical potential 
volt 
V =W/A 
Electric capacitance 
farad 
F =A s/V 
Electric resistance 
ohm 
Ω = V/A 
Frequency 
hertz 
Hz = s^{1} 
In the Système Interntional d’Unitès, SI, a unit is chosen for a particular purpose
and larger and smaller are obtained by applying a prefix to this unite and
multiplying or dividing by 10 or power of 10.
Larger units, obtained by multiplying by 1000, etc., are called ‘multiples’, smaller units, obtained by multiplying by 0.01, etc., are termed ‘submultiples’.
Multiplying factors
Factor 
Prefix 
Symbol 
1 000 000 000 [10^{9}]  giga  G 
1 000 000 [10^{6}] 
mega 
M 
1 000 [10^{3}] 
kilo 
k 
100 [10^{2}] 
hecto 
h 
10 [10 ] 
deca 
da 
0.1 [10^{1}] 
deci 
d 
0.01 [10^{2}] 
centi 
c 
0.001 [10^{3}] 
milli 
m 
0.000001 [10^{6}] 
micro 
μ 
Units outside the SI that are accepted for use with the SI
Derived quantity 
Unit symbol 
Value 
Minute 
min 
1 min = 60 s 
Hour 
h 
1 h = 60 min = 3600 s 
Day 
day 
1 day = 24 h = 86400 s 
Degree angle 
^{o} 
1^{o} = ^{ }(π/180) rad 
Liter 
l, (L) 
1 L = 1dm^{3} = 0.001 m^{3} 
Metric ton (tonne) 
t 
1 t = 1000 kg 
Other units outside the SI that are currently accepted for use with the SI
Derived quantity 
Unit symbol 
Value 
Nautical mile 

1 nautical mile = 1852 m 
Knot 

1 knot = 1.852 km/h 
Hectare 
ha 
1 ha = 1 hm^{2 }= 10^{4} m^{2} 
Bar 
bar 
1 bar = 0.1 MPa = 100 kPa = 1000 hPa = 10^{5} Pa 
SI unit rules and style conventions
 Unit symbols are placed after the
numerical value, leaving a space between the value and the symbol. For example
5 V not 5V.
 Only one prefix can be applied to a unit at a time. For example, 1000 kilonewton must not express 1 kilokilonewton but as meganewton; 0.001 millimeter must not be written as 1 millmillmeter but as 1 micrometer.
The correct use of upper and lower case letters (capital and small letters) is important. For example m meter, but M mega; k kilo, but K Kelvin.
 Symbols must not made plural by adding ‘s’ since ’s’ is symbol of second.
When a prefix is attached to a unit there should be no space between the prefix and the unit; for example, mm millimeter, kW kilowatt, MN meganewton.
When a complex unit is formed by multiplying two units together, however, the symbol should be separated by a space; for example, N m newton meter (the unit of torque).
 Abbreviations such as sec, cc, or mps are avoided and only standard unit symbols, prefix symbols, unit names, and prefix names are used, (s, cm^{3}, m/s).
 Unit symbols are generally written in lower case letters, except when the name of the unit is derived from a proper name. (Note that when the name of a unit which is derived from a proper name is written out in full, such as ampere or hertz, the name is not capitalized. The only exception to this is Celsius.)
Imperial unit
The Imperial units or the Imperial system is a collection of English units, first defined in the Weights and Measures Act of 1824, later refined (until 1959) and reduced. The units were introduced in the United Kingdom and its colonies, including Commonwealth countries, but excluding the then already independent United States.
Relation to other systems
The distinction between this imperial system and the U.S. customary units (also called standard units there) or older British/English units/systems and newer additions (footpoundsecond systems) is often not drawn precisely. Most length units are shared among the Imperial and U.S. systems, albeit partially and temporally defined slightly differently. Capacity measures differ the most due to the introduction of the Imperial gallon and the unification of wet and dry measures.
The term imperial should not be applied to English units that were outlawed in Weights and Measures Act of 1824 or earlier, or which had fallen out of use by that time, nor to postimperial inventions such as the slug or poundal.
Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other.
Imperial Units of length
1 inch = 25.4 mm
1 foot = 12 inches = 304.8 mm
1 yard = 3 feet = 36 inches = 914.4 mm
1 mile = 8 furlongs = 1760 yards = 5280 feet = 1.609 344 km
1 league = 3 miles = 5280 yards = 15840 feet = 4.828 032 km
Imperial Units of volume
1 fluid ounce (oz) = 0.028 413 062 5 L
1 pint = 20 oz = 0.568261 L
1 quart = 2 pint = 40 oz = 1.136522 L
1 gallon = 4 quart = 8 pints = 4.54609 L
1barrel = 36 gallon = 163.659 24 L
Imperial Units of weight and mass
1 ounce = 1/16 pound = 28.349 523 125 g
1 pound = 7000 grains = 453.592 37 g
1 ton = 20 hundredweight = 2240 pounds = 1016.046 908 8 kg
The USA ton (also known as the "short ton" is 2000 pounds, the UK ton (also known as the "long ton") is 2240 pounds, and the metric ton is 1000 kg.
The United States customary system (also called American system or, more rarely, "English units") is the most commonly used system of measurement in the United States. It is similar but not identical to the British Imperial units. The U.S. is the only industrialized nation that does not mainly use the metric system in its commercial and standards activities, although the International System of Units (SI, often referred to as "metric") is universally used in science, and increasingly in medicine, government, and many sectors of industry. The vast majority of U.S. customary units have been defined in terms of the meter and the kilogram since the Mendenhall Order of 1893 (and, in practice, for many years before that date). These definitions were refined in 1959.
The U.S. customary units have common roots with the Imperial units which were used in the British Empire. Many U.S. units are virtually identical to their Imperial counterparts, but the U.S. customary system developed from English units in use before the Imperial system was standardized in 1824, and there are several numerical differences from the Imperial system.
Units of volume
1 teaspoon (tsp) = 4.928921 mL
1 tablespoon (Tbsp) 3 tsp =14.78676 mL
1 US fluid ounce (fl oz) 2 Tbsp = 29.57353 mL
1 (liquid) US gallon (gal) = 3.785412 L
Units of temperature
Degrees Fahrenheit
are used in the United States to measure temperatures in most
nonscientific contexts. The Rankine scale of absolute temperature also saw some use in thermodynamics.
1 F = 9/5 C + 32
External links:
International System of Units (SI)
Definitions of the SI Units and Other Units of
Measurement
A Dictionary of Units of Measurement
http://www.unc.edu/~rowlett/units/index.html