I need some guidance...I am interested to calculate the current of Arduino Mega. Any body help or guide me how to calculate it.

Put a resistor (say 10ohms) in series with the positive (or the negative if you wish) supply line and measure the voltage drop across the resistor.

I (amps into your arduino) = V (voltage across resistor) x 10 (the value of the resistor)

The value of the resistor is of course chosen to give a reasonable voltage drop without 'starving' the load of operating voltage. 10ohms at say 100mA will produce 1 volt drop, which may be too large for your application, depending on minimum operating voltage and battery voltage. If you wish to measure higher currents, or can tolerate less voltage loss, then typically you may need to reduce the resistor accordingly to say 1 ohm, 0.1ohm or 0.01ohm.

This is because their is no easy way to calculate this value so you have to measure it.

I measured it a long time ago.

From memory, it was about 70mA.

Leo..