We present a measurement of the low-energy (0–60 keV) γ -ray spectrum produced in the α decay of ²³³U using a dedicated cryogenic magnetic microcalorimeter. The energy resolution of ∼ 10 eV , together with exceptional gain linearity, allows us to determine the energy of the low-lying isomeric state in ²²⁹Th using four complementary evaluation schemes. The most precise scheme determines the 229 Th isomer energy to be 8.10(17) eV, corresponding to 153.1(32) nm, superseding in precision previous values based on γ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be b₂₉ = 9.3(6)% and b₄₂ < 0.7%.