http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm

Impedance is equal to voltage divided by current. This means that a 4-ohm speaker draws more current than a 6-ohm speaker; more current gives it more power and usually enables it to play loudly more easily.
http://www.prestonelectronics.com/audio/Impedance.htm
It should be apparent by now that there is a relationship between pressure (voltage), flow (current) and restriction (impedance).

Example:

• Impedience (ohms?) = V/Current
• Current(Amps) = Voltage/Impedeince(ohms)
• On Little laptop speaker
• With volt meterd measured .004 x 2000 = 8
• BUT the device says 4 ohoms

Arduino puts out 5 volts when analog pins (8-13) are set to HIGH

• 4ohms = 5V/Xamps
• or
• 5V/4ohms = Xamps
• 4/1 = 5/x
• 4/5=x
• x=.8
• I need .8 amps of current

A very good calculator and aproximation of typical LED's if you do not have spec sheets:
http://led.linear1.org/1led.wiz
In the example:

• 5volts
• 3.3 forward voltage @20mA (recamended for 3-5mm LED's of a white color
• =APROXIMATELY a 100 ohm resistor at 1/8 Watts

LED calculation

• http://www.ehow.com/how_5865769_choose-resistors-12-volt-leds.html
• subtract forward voltage (assuming 3.3, as above)
• From Power supply 5
• =2.7
• Divide the result by current raiting (assuming 20map, and dont forget that
• 20mA is equal to .02 Amps
• 2.7/0.02=
• 135

Other, basic system:

• http://www.ehow.com/how_7810698_resistor-size-needed-electrical-circuit.html
• 5v/.02A=
• 250
• or if we are to asume voltage of device
• 3.3/.02=
• 165