I am going to use an ESP32 but in any case, i found this:
According to the ESP8266 datasheet, the ADC pin has 10 bit resolution. This means that your analog reading will return a value between 0 to 1024.
The ADC only converts voltage between 0 and 3.3Volts. So before you apply any voltage to your A0 pin, you need to make sure that you’re using a device that only outputs 3.3V. Otherwise you might need to use a voltage divider.
Note: with earlier versions of the ESP, the A0 pin operated at maximum voltage of 1.0V, so check your board A0 voltage before following the next circuit.