There are three different equations you can use to find the wattage rating for a resiter. If you use the correct values in the correct places, they will all give the same answer.

P = power measured in watts
E = voltage measured in volts
I = current measured in amps
R = resistance measured in ohms

Equation 1: P = R x I^2
You take the resitor value from the resitor equation and multiply it by current squared.

Equation 2: P = E x I
You take the voltage (that's voltage supply minus LED voltage forward, just like in the resistor equation) and multiply it by current.

Equation 3: It uses the values for E and R to find P, but I don't remember the equation. I'll have to look it up when I get home.