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I know how to use ohms law to calculate what size resistor I need but what I really need to know is how to tell/calculate what watt rating I need to use whether it be 1/4, 1/2, 1/8, 1,10 and so on.

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v=I*R
W=I*V
used W for watts instead of P for power, technically wrong but easier for some to understand.
Therefor by combining both equations
W = I squared * R
If you know the resistor value and the current flowing through it the just square the current value (in amps) and multiply by the resistor value (in ohms) to get the watts.

That all depends on the application. Lets say you take a 100 ohm resistor and place 9v through it to ground. With Ohms law we know that 0.09 A running through the circuit. So we take that 0.09A and multiply it by the 9V to get how much power is being used by the circuit. Which comes out to about .81W. So you will want to use at least a 1W resistor in that situation.

So you need to know how much current the circuit will be pulling so you can calculate the power used and you'll know what power rating of resistor you will need.