ACS712 Low Current Sensor
You would use a set of formulas like this:

$${ADC\_Value \over ADC\_MAX} \times ADC\_Volt\_Scale = ADC\_Voltage$$

You can see that this makes a ratio out of the read ADC value and the maximum value, then scales it by the maximum voltage that the ADC can read (this works for ADCs that start at 0V and go to some max voltage only!)

Now that you have the output voltage, you can offset it to zero, and divide the mV/mA conversion value:

$$(ADC\_Voltage - 2.5V) \div mV\_per\_mA\_CONST = Curent\_in\_mA$$

That should work in your case. If you have divider circuitry or other voltages in play, you might have to modify the formula to get what you want.