  The low-lying isomeric state of $^{229}$Th provides unique opportunities for
high-resolution laser spectroscopy of the atomic nucleus. We determine the
energy of this isomeric state by taking the absolute energy difference between
the excitation energy required to populate the 29.2-keV state from the
ground-state and the energy emitted in its decay to the isomeric excited state.
A transition-edge sensor microcalorimeter was used to measure the absolute
energy of the 29.2-keV $\gamma$-ray. Together with the cross-band transition
energy (29.2 keV$\to$ground) and the branching ratio of the 29.2-keV state
measured in a recent study, the isomer energy was determined to be
8.30$\pm$0.92 eV. Our result is in agreement with latest measurements based on
different experimental techniques, which further confirms that the isomeric
state of $^{229}$Th is in the laser-accessible vacuum ultraviolet range.
