SI units
SI units Systéme International d'Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units (a metric system based on the metre, kilogram, and second), SI units have now replaced c.g.s. units and Imperial units. The system has seven base units and two dimensionless units (formerly called supplementary units), all other units being derived from these nine units. There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lowercase letters). Decimal multiples of the units are indicated by a set of prefixes; when possible a prefix representing 10 raised to a power that is a multiple of three should be used.
TABLE 1.1 Base and dimensionless SI units
Physical quantity  Name  Symbol 

*dimensionless units  
length  metre  m 
mass  kilogram  kg 
time  second  s 
electric current  ampere  A 
thermodynamic temperature  kelvin  K 
luminous intensity  candela  cd 
amount of substance  mole  mol 
*plane angle  radian  rad 
*solid angle  steradian  sr 
TABLE 1.2 Derived SI units with special names
Physical quantity  Name of SI unit  Symbol of SI unit  

frequency  hertz  Hz  
energy  joule  J  
force  newton  N  
power  watt  W  
pressure  pascal  Pa  
electric  charge  coulomb  C 
electric potential difference  volt  V  
electric resistance  ohm  Ω  
electric conductance  siemens  S  
electric capacitance  farad  F  
magnetic flux  weber  Wb  
inductance  henry  H  
magnetic flux density (magnetic induction)  tesla  T  
luminous flux  lumen  lm  
illuminance  lux  lx  
absorbed dose  gray  Gy  
activity  becquerel  Bq  
dose equivalent  sievert  Sv 
TABLE 1.3 Decimal multiples and submultiples to be used with SI units
Submultiple  Prefix  Symbol  Multiple  Prefix  Symbol 

10^{−1}  deci  d  10  deca  da 
10^{−2}  centi  c  10^{2}  hecto  h 
10^{−3}  milli  m  10^{3}  kilo  k 
10^{−6}  micro  μ  10^{6}  mega  M 
10^{−9}  nano  n  10^{9}  giga  G 
10^{−12}  pico  p  10^{12}  tera  T 
10^{−15}  femto  f  10^{15}  peta  P 
10^{−18}  atto  a  10^{18}  exa  E 
10^{−21}  zepto  z  10^{21}  zetta  Z 
10^{−24}  yocto  y  10^{24}  yotta  Y 
TABLE 1.4 Conversion of units of SI units
From  To  Multiply by 

in  m  2.54 × 10^{−2} 
ft  m  0.3048 
sq. in  m^{2}  6.4516 × 10^{−4} 
sq. ft  m^{2}  9.2903 × 10^{−2} 
cu. in  m^{3}  1.63871 × 10^{−5} 
cu. ft  m^{3}  2.83168 × 10^{−2} 
l(itre)  m^{3}  10^{−3} 
gal(lon)  l(itre)  4.546 09 
miles/hr  m s^{−1}  0.477 04 
km/hr  m s^{−1}  0.277 78 
lb  kg  0.453 592 
g cm^{−3}  kg m^{−3}  10^{3} 
lb/in^{3}  kg m^{−3}  2.767 99 × 10^{4} 
dyne  N  10^{−5} 
poundal  N  0.138 255 
lbf  N  4.448 22 
mmHg  Pa  133.322 
atmosphere  Pa  1.013 25 × 10^{5} 
hp  W  745.7 
erg  J  10^{−7} 
eV  J  1.602 10 × 10^{−19} 
kW h  J  3.6 × 10^{6} 
cal  J  4.1868 
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SI units
SI units (Système International d'Unités) pl. n. the internationally agreed system of units, based on the metrekilogramsecond system, now in use for all scientific purposes.
Table 11.1 Base and supplementary SI units
Physical quantity  Name of unit  Symbol for unit 

length  metre  m 
mass  kilogram  kg 
time  second  s 
electric current  ampere  A 
thermodynamic temperature  kelvin  K 
luminous intensity  candela  cd 
amount of substance  mole  mol 
*plane angle  radian  rad 
*solid angle  steradian  sr 
*supplementary units 
Table 11.2 Derived SI units with special names
Physical quantity  Name of unit  Symbol for unit 

frequency  hertz  Hz 
energy  joule  J 
force  newton  N 
power  watt  W 
pressure  pascal  Pa 
electric charge  coulomb  C 
electric potential difference  volt  V 
electric resistance  ohm  Ω 
electric conductance  siemens  S 
electric capacitance  farad  F 
magnetic flux  weber  Wb 
inductance  henry  H 
magnetic flux density (magnetic induction)  tesla  T 
luminous flux  lumen  lm 
illuminance (illumination)  lux  lx 
absorbed dose  gray  Gy 
activity  becquerel  Bq 
dose equivalent  sievert  Sv 
Table 11.3 Decimal multiples and submultiples to be used with SI units
Submultiple  Prefix  Symbol  Multiple  Prefix  Symbol 

10^{−1}  deci  d  10^{1}  deca  da 
10^{−2}  centi  c  10^{2}  hecto  h 
10^{−3}  milli  m  10^{3}  kilo  k 
10^{−6}  micro  μ  10^{6}  mega  M 
10^{−9}  nano  n  10^{9}  giga  G 
10^{−12}  pico  p  10^{12}  tera  T 
10^{−15}  femto  f  10^{15}  peta  P 
10^{−16}  atto  a  10^{16}  exa  E 
10^{−21}  zepto  z  10^{21}  zetta  Z 
10^{−24}  yocto  y  10^{24}  yotta  Y 
Table 11.4 Conversion of units to and from SI units
From  To  Multiply by 

Temperature conversion  
°C (Celsius) = 5/9(°F − 32)  
°F (Fahrenheit) = (9/5 × °C) + 32  
in  m  0.0254 
ft  m  0.3048 
sq in  m^{2}  0.00064516 
sq ft  m^{2}  0.092903 
cu in  m^{3}  0.0000164 
cu ft  m^{3}  0.0283168 
l(itre)  m^{3}  0.001 
gal(lon)  m^{3}  0.0045609 
gal(lon)  litres  4.5609 
lb  kg  0.453592 
g cm^{−3}  kg m^{−3}  1000 
lb/in^{3}  kg m^{−3}  27679.9 
mmHg  Pa  133.322 
cal  J  4.1868 
m  in  39.3701 
cm  in  0.393701 
cm^{2}  sq in  0.155 
m^{2}  sq in  1550 
m^{2}  sq ft  10.7639 
m^{3}  cu in  61023.6 
m^{3}  cu ft  35.3146 
m^{3}  l(itre)  1000 
m^{3}  gal(lon)  219.969 
kg  lb  2.20462 
kg m^{−3}  g cm^{−3}  0.001 
kg m^{−3}  lb/in^{3}  0.0000363 
Pa  mmHg  0.0075006 
J  cal  0.238846 
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SI units
SI units (Système International d'Unités) The international system of units of measurement which evolved over a number of years from the m.k.s. system (based on the metre, kilogram, and second) and was finally adopted in 1960 by the 11th Conférence Générale des Poids et Mesures. The system is widely used in trade and is recommended for all scientific work. There are seven basic units: metre (m, length); kilogram (kg, mass); second (s, time); ampere (A, electric current); kelvin (K, thermodynamic temperature); candela (cd, luminous intensity); and mole (mol, amount of substance). There are also two supplementary units: radian (rad, plane angle); and steradian (sr, solid angle). From these, a further 18 units are derived: hertz (Hz, frequency); joule (J, energy); newton (N, force); watt (W, power); pascal (Pa, pressure); coulomb (C, electric charge); volt (V, electric potential); ohm (Ω, electric resistance); siemens (S, electric conductance); farad (F, electric capacitance); weber (Wb, magnetic flux); tesla (T, magnetic induction); lumen (lm, luminous flux); lux (lx, illuminance); gray (Gy, absorbed radiation dose); becquerel (Bq, radioactivity); and sievert (Sv, radiation dose equivalent). See Derived SI units.
SI Units (Système international d'unités)
Quantity  Name of unit  Symbol  Equivalent  Reciprocal 

length  metre  m  3.281 feet  1 ft = 0.3048 m 
mass  kilogram  kg  2.2 pounds  1 lb = 0.454 kg 
time  second  s  
electric current  ampere  A  
thermodynamic temperature  kelvin  K  1°C = 1.8°F  1°C = 1 K 
luminous intensity  candela  cd  
amount of substance  mole  mol 
Supplementary units
Quantity  Unit  Symbol 

plane angle  radian  rad 
solid angle  steradian  sr 
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SI units
SI units (Système International d'Unités)
Quantity  Name of unit  Symbol  Equivalent  Reciprocal 

length  metre  m  3.281 feet  1 ft = 0.3048 m 
mass  kilogram  kg  2.2 pounds  1 lb = 0.454 kg 
time  second  s  
electric current  ampere  A  
thermodynamic temperature  kelvin  K  1°C = 1.8°F  1°C = 1 K 
luminous intensity  candela  cd  
amount of substance  mole  mol 
Supplementary units
Quantity  Unit  Symbol 

plane angle  radian  rad 
solid angle  steradian  sr 
Derived SI units
Quantity  Name of unit  Symbol  Equivalent  Reciprocal 

frequency  hertz  Hz  
energy  joule  J  0.2388 calories  1 cal = 4.1868 J 
force  newton  N  0.225 pounds force  1 lbf = 4.448 N 
power  watt  W  0.00134 horse power  1 hp = 745.7 W 
pressure  pascal  Pa  0.00689 pounds force/sq.inch  1 lbf/sq.in = 145 Pa 
electric charge  coulomb  C  
electric potential difference  volt  V  
electric resistance  ohm  Ω  
electric conductance  siemens  S  
electric capacitance  farad  F  
magnetic flux  weber  Wb  
inductance  henry  H  
magnetic flux density  tesla  T  
luminous flux  lumen  lm  
illuminance  lux  lx  
absorbed dose  gray  Gy  
activity  becquerel  Bq  
dose equivalent  sievert  Sv 
Multiples used with SI units
Name of multiple  Symbol  Value (multiply by) 

atto  a  10^{−18} 
femto  f  10^{−15} 
pico  p  10^{−12} 
nano  n  10^{−9} 
micro  μ  10^{−6} 
milli  m  10^{−3} 
centi  c  10^{−2} 
deci  d  10^{−1} 
deca  da  10 
hecto  h  10^{2} 
kilo  k  10^{3} 
mega  M  10^{6} 
giga  G  10^{9} 
tera  T  10^{12} 
peta  P  10^{15} 
exa  E  10^{18} 
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SI units
SI units (Système International d'Unités)
Quantity  Name of unit  Symbol  Equivalent  Reciprocal 

length  metre  m  3.281 feet  1 ft = 0.3048 m 
mass  kilogram  kg  2.2 pounds  1 lb = 0.454 kg 
time  second  s  
electric current  ampere  A  
thermodynamic temperature  kelvin  K  1°C = 1.8°F  1°C = 1 K 
luminous intensity  candela  cd  
amount of substance  mole  mol 
Supplementary units
Quantity  Unit  Symbol 

plane angle  radian  rad 
solid angle  steradian  sr 
Derived SI units
Quantity  Name of unit  Symbol  Equivalent  Reciprocal 

frequency  hertz  Hz  
energy  joule  J  0.2388 calories  1 cal = 4.1868 J 
force  newton  N  0.225 pounds force  1 lbf = 4.448 N 
power  watt  W  0.00134 horse power  1 hp = 745.7 W 
pressure  pascal  Pa  0.00689 pounds force/sq.inch  1 lbf/sq.in = 145 Pa 
electric charge  coulomb  C  
electric potential difference  volt  V  
electric resistance  ohm  Ω  
electric conductance  siemens  S  
electric capacitance  farad  F  
magnetic flux  weber  Wb  
inductance  henry  H  
magnetic flux density  tesla  T  
luminous flux  lumen  lm  
illuminous  lux  lx  
absorbed dose  gray  Gy  
activity  becquerel  Bq  
dose equivalent  sievert  Sv 
Multiples used with SI units
Name of multiple  Symbol  Value (multiply by) 

atto  a  10^{−18} 
femto  f  10^{−15} 
pico  p  10^{−12} 
nano  n  10^{−9} 
micro  μ  10^{−6} 
milli  m  10^{−3} 
centi  c  10^{−2} 
deci  d  10^{−1} 
deca  da  10 
hecto  h  10^{2} 
kilo  k  10^{3} 
mega  M  10^{6} 
giga  G  10^{9} 
tera  T  10^{12} 
peta  P  10^{15} 
exa  E  10^{18} 
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SI units
SI units (Système International d'Unites) Internationally agreed system of units, derived from the mks (metre, kilogram, and second) system. SI units are now used for many scientific purposes, and have replaced the fps (foot, pound, and second) and cgs (centimetre, gram, and second) systems. The seven basic units are: the metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electrical current, Kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity.
Measurement  Unit  Symbol 

Basic units  
Length  metre  m 
Mass  kilogram  kg 
Time  second  s 
Electric current  ampere  A 
Thermodynamic temperature  kelvin  K 
Amount of substance  mole  mol 
Luminous intensity  candela  cd 
Supplementary units  
Plane angle  radian  rad 
Solid angle  steradian  sr 
Derived units  
Frequency  hertz  Hz 
Force  newton  N 
Pressure, stress  pascal  Pa 
Work (energy, heat)  joule  J 
Power  watt  W 
Electric charge  coulomb  C 
Electromotive force  volt  V 
Electric resistance  ohm  1/2 
Electric conductance  siemens  S 
Electric capacitance  farad  F 
Inductance  henry  H 
Magnetic flux  weber  Wb 
Magnetic flux density  tesla  T 
Illuminance  lux  lx 
Luminous flux  lumen  lm 
Radiation exposure  röntgen  r 
Radiation activity  becquerel  Bq 
Radiation absorbed dose  gray  Gy 
Radiation dose equivalent  sievert  Sv 
Celsius temperature  °Celsius  °C 
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SI units
SI units The system of units of measurement adopted internationally for scientific and technical use under the Système International d'Unités. There are seven fundamental units – the meter, second, kilogram, ampere, kelvin, candela, and mole – of seven dimensionally independent physical quantities. Other units are derived algebraically from these base units without the use of numerical factors, or are dimensionless (like the radian). The symbols for all units are standardized, as are the prefixes (and their symbols) that represent decimal multiples (e.g. nano, micro, milli, kilo, mega, giga) of the units.
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