Common Units and Prefixes used in Physical Systems



The following table contains the fundamental units for various types of systems.  See also the page at the NIST (National Institute of Standards and Technology) website.  Each unit is written out, followed by its abbreviation in parentheses, ().  If other equivalent forms of the unit are commonly used, they are given in brackets, [].  Both SI (from French for "Le Système International d'Unités") and English units are given.


Fundamental Quantities   SI unit English Unit   Conversion
Time second (s) second (s)  
Mass kilogram (kg) slug 1 slug = 14.6 kg
Length meter (m) foot (ft) 1 ft = 0.305 m
Force Newton (N) pound (lb) 1 lb = 4.45 N
Energy Joule (J) [W-s, N-m] foot-pound (ft-lb)
British Thermal Unit (BTU)
1 ft-lb = 1.36 J
1 BTU = 1055 J
Power Watt (W) [J/s] horsepower (hp) 1 hp = 746 W
Moment of intertia (kg-m²) (slug-ft²) 1 slug-ft² = 1.36 N-m²
Torque Newton-meter (N-m) foot-pound (ft-lb) 1 ft-lb = 1.36 N-m
Voltage Volt (V) Volts (V)  
Charge Coulomb (C) Coulomb (C)  
Current Amperes (A=C/s) Amperes (A)  
Absolute Temperature  Degrees Kelvin (°K) Degrees Rankine (°R) 1 °R = 9/5 °K
Degrees Celsius (°C) Degrees Fahrenheit (°F) °F = °Cx9/5+32
°C = °K-273.15
°F = °R-459.67
Linear Mechanical Systems  
Spring Constant (N/m) (lb/ft)  
Friction Coefficient  (N-s/m) (lb-s/ft)  
Rotating Mechanical Systems  
Spring Constant (N-m/rad) (ft-lb/rad)  
Friction Coeffiicient (N-m-s/rad) (ft-lb-s/rad)  
Thermal Systems  
Thermal Resistance °K/W s-°R/BTU  
Thermal Conductance W/°K BTU/(s-°R)  
Thermal Capacitance, or
Thermal Mass
J/°K BTU/°R  
Electrical Systems  
Electrical Resistance Ohm-Ω (V/A) Ohm-Ω (V/A)  
Electrical Conductance Siemens-S (A/V) (Note) 

You will sometimes see conductance measured with a unit of a "mho". The word "mho"is the word "ohm" spelled backwards); and the symbol was an upside down Ω (℧) because conductance is the inverse of resistance.

Siemens-S (A/V)  
Capacitance Farad-F (C/V) Farad-F (C/V)  
Inductance Henry-H (V-s/A) Henry-H (V-s/A)  


The following table gives some of the more common SI prefixes.  If the prefix precedes the name of a unit in the above table, multiply the unit by the prefix multiplier.  For example 4.2 milliAmperes (or 4.2 mA) is 4.2·10-3A (ref with full list).  (note: there is an online effort to have "hella" named as the prefix for 10+27.  It has been adopted by google.)


Prefix Quantities   Symbol Multiplier  
pico p 10-12
nano n 10-9
micro μ 10-6
milli m 10-3
centi c 10-2
deci d 10-1
kilo k 10+3
mega M 10+6
giga G 10+9
tera T 10+12