ESP8266 Community Forum

trackerj wrote:These ESP8266 internal ADC related articles might help you :

ESP8266 Internal ADC explained
ESP8288 Internal ADC - the easy way example
ADC frontend example

Thanks! Its seems development boards have a voltage divider on the ADC but the bare bones ESP01, ESP12 don't. I added a 100k potentiometer which made it calibratable.

ESP8266 ADC operates between 0V to 1V so 0V=0ADC and 1V=1023ADC.
0.7V= 716ADC
Voltage=ADC*1/1024