We report the effect of lateral designs and resultant parasitic thermal conductance on the absolute accuracy of microcalorimeters based on a silicon-nitride membrane. Thermal properties are quantified by the curve fitting scheme based on the thermal relaxation method with two thermal decay times. In the studies of various microcalorimeter layouts, the temperature dependence of errors in the absolute values of heat capacity was found to be approximately proportional to that of the thermal conductance between the isothermal platform and the temperature reservoir. Decreasing parasitic thermal conductance by modifying the lateral designs for the thermometer and heater allowed for absolute accuracies that were typically less than 5% in a temperature range between 20 and 300 K. (C) 2008 Elsevier B.V. All rights reserved.