In measuring thermophysical properties using the transient calorimetric method, conduction heat leak from test specimens can be a source of error if not properly accounted for. An analysis method is presented in this paper for determining the magnitude of this leak. The analysis consists of deriving the temperature profiles in the specimen support. These temperatures are then used to determine the heat leak using two different approaches. In the first approach, the heat penetration depth in the support is derived and used to obtain an added thermal capacitance to the specimen. In the second approach, the heat leak was considered to reduce the net heat input. Both small and large heat‐leak cases were analyzed, with the latter estimated by means of two recurrence relations. A chart was also developed for a rapid estimation of this leak. In the absence of a reliable experiment to measure the heat leak, this paper provides a viable alternative for accurate leak determination.