The Medhurst formula gives the self-capacity of a coil in picofarads [pF]. It has the following form:

C [pF] = K . D [cm]

where K is the Medhurst coefficient and D, the coil's diameter in centimeters.

K values are a function of the aspect-ratio ar i.e. the height/diameter (H/D) coil.

Searching the web and technical books about this topic I've come across the following values, which are shown below. Range of ar is 1 to 10 (common TC secondary values).

Medhursts Original Papers published in Wireless Engineer 1947