A simple analytical relationship for theQof a resonant spherical cavity with a central droplet is derived assuming superconducting cavity walls. In this caseQ = (&ohgr; × stored energy)/( (power loss)where power loss refers to the energy rate delivered to the central droplet. It is assumed that the skin depth&dgr;is small compared with the droplet radiusa, and that the wavelength,&lgr; = 2&pgr;c/&ohgr;, be small compared with the cavity radiusb, and large compared with the droplet radius. The equation isQ = (bc2) / (a2&dgr;&ohgr;2). It is shown that in the case of conducting cavity walls a short wavelength minimizes the wall losses.