Robin2:
Except that Atmel was not brave enough to explain (in the Atmega 328 datasheet at least) how to convert an ADC reading back to the voltage from which the reading was obtained.I don't think any of the subsequent comments have improved on @CodingBadly's Reply #75
...R
No, really, really RTFM. As in, download the datasheet, and read it.
The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1 LSB.