Basic & Derived SI Units

Basic SI Units

Units in combination with other units

There are SI units for many any and all useful physical quantities, of which just a few are given in the table below. Importantly, the SI system is a system. If you put SI values into a formula for all quantities, the answer always comes out in SI units. 

On the Internet, it is not easy to express superscripts without using pictures that take too much time to load.  Therefore, we use a caret to express the exponent.  For example, we use m^2 to represent square meters. 

 

NAME
DEFINITION
UNIT
Area 
 length ´  width 
volume 
length ´  width ´  height 
velocity 
distance ¸ time 
m/s 
acceleration 
velocity change ¸ time 
force 
push or pull = mass ´  acceleration 
energy 
capacity to do work W = force ´  distance 
power 
energy ¸ time 
J/s = watt = W 
intensity (solar) 
power ¸ area 
 
 
It is important to remember that energy is measured in joules, and power is measured in watts. One joule = 1 watt ´ 1 second. People usually associate the watt with electricity, but that is because the SI system has traditionally been used only for electricity in the US. The watt is a general unit for power --- energy divided by time --- regardless of whether electricity is involved. 

The so-called power company sends you a bill for energy (not power), but it uses a non-SI unit for time. To get the energy, they multiply kilowatts by hours to get kWh. 
 

 For more information, see National Institute for Standards & Technology (NIST)
 

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