Debris thermal conductivity is a critical parameter in calculating a glacier’s sub- debris ice melt. The method widely used in publications to calculate the apparent thermal conductivity of supraglacial debris layers is based on an estimate of volumet- ric heat capacity of the debris and simple heat diffusion principles. It is presented in Conway and Rasmussen (2000). The analysis of heat diffusion requires a vertical array of temperature measurements through the supraglacial debris cover. This study explores the effect of the temporal and spatial sampling interval and other error sources on the thermal diffusivity values derived using this method. Results show that temporal and spatial truncation errors all underestimate values of thermal diffusivity with increasing sampling intervals. Also, the thermistor precision, the diurnal temperature cycle, and vertical thermistor displacement result in systematic errors. Therefore great care must be taken when performing and analyzing the debris layer measurements to produce reliable results. This problem especially becomes apparent because only a fraction of the available datasets are suitable for determining reliable thermal diffusivity values. Therefore, in this work, an online web application was developed to help other scientists investigate the effect of the sampling interval on their calculated sub-debris ice melt and better plan future measurement campaigns. This study falls under the remit of the IACS working group on debris-covered glaciers and is supported by data provided from within this group.