The No Thermal category detects drills lacking thermals and, according to their size, report the total copper area associated with them.
The measurement value is calculated as <number of thermal spokes> * <width of spokes> * <layer thickness> (as found in the layer attribute .copper_weight * 1.35 [the thickness of 1 ounce of copper]).
If you work in metric units, multiply the number obtained for layer thickness by the translation factor 25.4.
The width of a spoke is measured from one side to the opposite side of the spoke gap. Measurements are cumulative.

Identified thermals are symbols of 3, 4, or 5 spokes. Thermals of more than five spokes, unsymmetrical thermals, or drawn thermals are not always recognized and are reported to category ‘No Thermal’.