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Calculating the power dissipation of an LDO is very simple: You simply need to know Vin (voltage suppled TO the LDO), Vout (voltage suppled FROM the LDO), and Iout (current suppled by the LDO). The power dissipated will simply be (Vin - Vout) * Iout.
If you want a more precise calculation (this becomes useful for calculating the power loss during standby), you should take into account the LDO's quiescent current (the current it draws regardless of the output current; this is typically called Iq). Then the power dissipation becomes:
Pw = (Vin - Vout) * Iout + Vin * Iq.
Typically, Vin * Iq is a very small amount of power, so when you're drawing lots of current it becomes negligible compared with (Vin - Vout) * Iout. Under heavy loads, Vin * Iq can be ignored.