parasitic coil capacitance from frequency

[Zak28]

Sweeping a coil with generator measured a coils voltage peaking at ~30.7Mhz and dipping at ~14Mhz - its inductance is ~4mH.

Does this mean its parasitic capacitance is 6.71 fempto farads?

[FvM]

Can't be answered without knowing the complete measurement circuit.

[CataM]

Another way is to apply fast rising edge step voltage and measure the ringing.

[FvM]

Parasitic coil capacitance refers to parallel capacitance, I presume. Thus you are looking for the parallel resonance of the unloaded coil. The best approach for unloaded resonance measurement is a weakly coupled grid dip meter or an equivalent setup.

The fact that your measurement shows two resonances makes clear that your setup isn't a simple LC parallel circuit but some kind of LCC or LLC circuit. And most likely parasitic parameters of the measurement setup are falsifying the original resonance frequency.