  By analyzing 2.93 fb$^{-1}$ of data collected at $\sqrt s=3.773$ GeV with the
BESIII detector, we measure the absolute branching fraction ${\mathcal
B}(D^{+}\rightarrow\bar K^0\mu^{+}\nu_{\mu})=(8.72 \pm 0.07_{\rm stat.} \pm
0.18_{\rm sys.})\%$, which is consistent with previous measurements within
uncertainties but with significantly improved precision. Combining the Particle
Data Group values of ${\mathcal B}(D^0\to K^-\mu^+\nu_\mu)$, ${\mathcal
B}(D^{+}\rightarrow\bar K^0 e^{+}\nu_{e})$, and the lifetimes of the $D^0$ and
$D^+$ mesons with the value of ${\mathcal B}(D^{+}\rightarrow\bar K^0
\mu^{+}\nu_{\mu})$ measured in this work, we determine the following ratios of
partial widths: $\Gamma(D^0\to K^-\mu^+\nu_\mu)/\Gamma(D^{+}\rightarrow\bar
K^0\mu^{+}\nu_{\mu})=0.963\pm0.044$ and $\Gamma(D^{+}\rightarrow\bar K^0
\mu^{+}\nu_{\mu})/\Gamma(D^{+}\rightarrow\bar K^0 e^{+}\nu_{e})=0.988\pm0.033$.
