The extent of subsurface flow and recharge areas of thermal groundwater systems are difficult to estimate. Two simple methods for making such estimates are described. The methods are based on heat flow and on the 14C age of thermal groundwater discharge. The two methods were applied to thermal groundwater systems in the southern Idaho Batholith and the results suggest that although thermal groundwater only accounts for about 1.5% of the drainage basin base flow, about one-third of the drainage basin surface area is involved in thermal groundwater system recharge. The results also suggest that hard rock hydrothermal heating does not simply occur in discrete fault and fracture systems, but rather in the vast volume of granitic mass. In the southern Idaho Batholith the total thermal system flow area is ∼2100 km3 of granite and the fault and fracture systems are the final collectors of thermal groundwater prior to surface discharge. The methods presented rely on readily obtainable data for most of hydrothermal systems.