Conversion of units

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Name of unitSymbolRelation to SI units
length, l
metre (SI base unit)m 
bohr (au)a0, b~ 5.291 77 x 10-11 m
ångströmÅ= 10-10 m
micronμ= μm = 10-6 m
x unitX~ 1.002 x 10-13 m
inchin= 2.54 x 10-2 m
footft= 12 in = 0.3048 m
yardyd= 3 ft = 0.9144 m
milemi= 1760 yd = 1609.344 m
nautical mile = 1852 m = 6076.1 feet
geographical mile = 1855 m = 6087.15 feet
astronomical unitAU= 1.496 00 x 1011 m
parsecpc~ 3.085 68 x 1016 m
light yearl.y.~ 9.460 528 x 1015 m
light second 299 792 458 m
area, A
square metre (SI unit)m2 
barnb10-28 m2
acre ~ 4046.856 m2
area= 100 m2
hectareha= 104 m2
volume, V
cubic metre (SI unit)m3 
litrel, L= dm3 = 10-3 m3
lambdaλ= μl = 10-6 dm3
barrel (US) ~ 158.987 dm3
gallon (US)gal (US)= 3.78541 dm3
gallon (UK)gal (UK)= 4.546 09 dm3
mass, m
kilogram (SI base unit)kg 
electron mass (au)me~ 9.109 39 x 10-31 kg
unified atomic mass unit, daltonu, Da~ 1.660 540 x 10-27 kg
gammaγ= μg
tonnet= 103 kg
pound (avoirdupois)lb= 0.453 592 37 kg
ounce (avoirdupois)oz~ 28.3495 g
ounce (troy)oz (troy)~ 31.1035g
graingr= 64.798 91 mg
time, t
second (SI base unit)s 
au of time ~ 2.418 88 x 10-17 s
minutemin= 60 s
hourh= 3600 s
dayd= 86 400 s (not exactly defined due to 'leap' seconds
yeara~ 31 556 952 s
svedbergSv= ~ 10-13 s
acceleration, a
(SI unit)m.s-2 
standard acceleration of free fallgn= 9.806 65 m.s-2
galileoGal= 10-2 m.s-2
force, F
newton (SI unit)N= kg.m.s-2
dyne (cgs unit)dyn= 10-5N
au of force ~ 8.238 73 x 10-8N
pound-forcelbf= 4.448 22 N
kilogram-forcekgf= 9.806 65 N
energy, U
joule (SI unit)J = N.m= kg.m2.s-2
erg (cgs unit)erg= 10-7 J
hartee (au)Eh~4.359 75 x 10-18 J
rydbergRy~ 2.179 87 x 10-18 J
electronvolteV= e x V ~ 1.602 18 x 10-19 J
calorie, thermochemicalcalth= 4.184 J
calorie, internationalcalIT= 4.1868 J
15 oC caloriecalIT~ 4.1855 J
litre atmospherel atm= 101.325 J
British thermal unitBtu= 1055.06 J
pressure, p
pascal (SI unit)Pa= N.m-2 = kg.m-1.s-2
atmosphereatm= 101 325 Pa
barbar= 105 Pa
torrTorr= (101 325/760) Pa ~ 133.322 Pa
millimetre of mercury (conventional)mmHg= 13.5951 x 980.655 x 10-2 Pa ~ 133.322 Pa
pounds per square inchpsi~ 6.894 757 x 103 Pa
power, P
watt (SI unit)W= J.s-1 = N.m.s-1 = kg.m2.s-3
horse powerhp=745.7 W
action, angular momentum, L, J
SI unitJ.s. = N.m.s= kg.m2.s-1
cgs uniterg.s= 10-7 J.s
au of actionh= h/2π = 1.054 57 x 10-34 J.s
dynamic viscosity, η
SI unitPa.s = N.m-2.s= kg.m-1.s-1
poiseP= 10-1Pa.s
centipoisecP= mPa.s
kinematic viscosity, ν
SI unit m2.s-1  
stokesSt= 10-4m2.s-1
thermodynamic temperature, T
kelvin (SI base unit)K 
degree CelsiusoCT in oC = T in K - 273.15
degree FahrenheitoFT in oF = 1.8 (T in oC) + 32
degree RankineoR= (5/9) K


A useful online converter is at

http://www.sciencemadesimple.com/conversions.html

The Unix program units is a useful tool for units conversion.