Volume Bragg gratings (VBGs) in photothermorefractive (PTR) glass are widely used for laser beam control including high-power laser systems. Among them, spectral beam combining based on VBGs is one of the most promising. Achieving $100+\mathrm{kW}$ of combined laser beams requires the development of PTR glass and VBGs with an extremely low absorption coefficient and therefore methods of its measurement. This paper describes the calorimetric method that was developed for measuring a low absorption coefficient in PTR glass and VBGs. It is based on transmission monitoring of the intrinsic Fabry–Perot interferometer produced by the plane-parallel surfaces of the measured optical elements when heated by high-power laser radiation. An absorption coefficient at $1085\mathrm{nm}$ as low as $5\times {10}^{-5}{\mathrm{cm}}^{-1}$ is demonstrated in pristine PTR glass while an absorption coefficient as low as $1\times {10}^{-4}{\mathrm{cm}}^{-1}$ is measured in high-efficiency reflecting Bragg gratings with highest purity. The actual level of absorption in PTR glass allows laser beam control at the $10\mathrm{kW}$ level, while the $100\mathrm{kW}$ level would require active cooling and/or decreasing the absorption in PTR Bragg gratings to a value similar to that in virgin PTR glass.