ESP32-C6 ADC readings disagree with a multimeter: what does calibration fix?

Check the chip's allowed input range and the selected attenuation first. Use the calibration scheme supported by the chip and ESP-IDF version to convert raw readings to estimated millivolts. Calibration is not permission to exceed the pin rating, and an input setting from another ESP32 variant may not apply.

For a divider, reconstruct the source as V_source = V_adc × (R_top + R_bottom) / R_bottom. With 30 kΩ above 10 kΩ, the nominal multiplier is four. Actual resistor tolerance, leakage, source impedance and input loading contribute errors that an ADC calibration routine cannot remove.

Compare several known DC points across the intended range and log spread as well as mean error. Repeat with radios active and inactive to look for supply or grounding effects. Averaging can reduce some noise, but it does not correct clipping, an incorrect divider ratio or systematic offset.