  The correct numerical calculation of the resonance characteristics and,
principally, the quality factor $Q$ of contemporary photonic and plasmonic
resonant systems is of utmost importance, since $Q$ defines the bandwidth and
affects nonlinear and spontaneous emission processes. Here, we comparatively
assess the commonly used methods for calculating $Q$ using spectral simulations
with commercially available, general-purpose software. We study the
applicability range of these methods through judiciously selected examples
covering different material systems and frequency regimes from the far-infrared
to the visible. We care in highlighting the underlying physical and numerical
reasons limiting the applicability of each one. Our findings demonstrate that
in contemporary systems (plasmonics, 2D materials) $Q$ calculation is not
trivial, mainly due to the physical complication of strong material dispersion
and light leakage. Our work can act as a reference for the mindful and accurate
calculation of the quality factor and can serve as a handbook for its
evaluation in guided-wave and free-space photonic and plasmonic resonant
systems.
