This paper presents a simplified method to calculate heat loss from slab-on-grade buildings. To develop a simplified method, a transient finite-difference model for heat loss from slab-on-grade buildings was developed. Based on results from the finite difference model, a simplified model of heat loss as a function of parallel thermal resistances and the difference between indoor air temperature and an effective ground temperature is proposed. The simplified method predicts heat loss using the weighted average of the annual average temperature and the average temperature of the past three months. Useof this effective ground temperature in the simplified model results in good agreement between heat loss predicted by the finite difference and simplified model in multiple locations. In addition, good agreement was found between the effective ground temperature and Kasuda's equation for ground temperature at a depth of 4.57 m (15 ft). A correction factor is added to the simplified model to account for the heat loss from the ends of the building. Heat loss from the simplified model and ASHRAE (2009) method is compared with reasonable agreement. The simplified method proposed here has the advantage of directly using actual temperature data as the driver for heat loss, rather than assuming that heat loss follows an annual sinusoidal function, and thus more accurately accounts for variations in heat loss caused by temperature irregularities.