We describe a laser calorimeter for measuring the absorption of an optical coating deposited on a thin disc. The sample with diameter 2.54cm and thickness between 0.02 to 0.13cm is irradiated with 6 watt CW Nd:YAG laser. The temperature of the coated disc is compared to that of a reference disc using thermistors in mechanical contact with the samples. Measurements are made under vacuum (100 torr) to reduce temperature variations caused by convection. The calorimeter was evaluated by measuring coated and uncoated discs of various thicknesses with absorptances from 9 × 10^-6 to 5 × 10^-3. These measurements demonstrated a repeatability of ± 2% at an absorptance level of 1 × 10^-4 and a sensitivity for a 0.04cm thick substrate of 3.8 × 10⁵ µV/watt. The lower absorption limit of the calorimeter is 2 × 10^-6. The results of measurements on polished, uncoated fused silica discs with thicknesses from 0.04 to 0.13cm show a surface absorptance of 6 × 10^-6 and a bulk value of 5 × 10^-5/cm.