A directional slug calorimeter is provided to enable accurate non-isotropic heat flux measurement in a high-temperature environment. The slug calorimeter includes a tubular member; a plurality of slots formed along a portion of an axial length of the tubular member, insulation disposed within the tubular member and within the plurality of slots; and thermocouples disposed with a thermocouple junction. The plurality of slots divide the tubular member into discrete segments. The thermocouples are disposed in a central region of each of the discrete segments.