The extreme pressures and temperatures of the gas produced by detonating a High Explosive (HE) make it difficult to use experimental measurements to estimate the Equation Of State (EOS), the physics model that relates pressure, temperature, and density of the gas. Instead of measuring pressure directly one measures effects like the acceleration of metals driven by the HE. Typically one fits a few free parameters in a fixed functional form to measurements from a single experiment. The present work uses the optimization tool F_UNCLE to incorporate data from multiple experiments into a single EOS model for the gas produced by detonating the explosive PBX 9501. The model is verified by comparison to an experiment from outside the set of calibration data. The uncertainty in the EOS is also is examined to determine how each calibration experiment constrains the model and how the uncertainty arising from all calibration experiments affects predictions. This work identifies an EOS for HE detonation products and uncertainty about the EOS.