We define the confinement factor for a subwavelength resonator, present a simple approach to calculate it numerically, and demonstrate that by using a specific resonator based on a silver rod coated with active material. Unlike the common approach of using the profile of the resonant electromagnetic mode, our formulation relies on using the characteristic equation for the complex frequency with inclusion of damping due to Joule and radiative losses. This removes an arbitrariness related to the definition of the mode volume in subwavelength resonators. Our analysis of a coated silver rod suggests that there is an optimal value for the active region thickness. The confinement factors can be comparable to that in Fabry–Perot resonators, radiative losses are comparable or exceed Joule losses and the threshold gain is high.