The accurate estimation of groundwater head in tunnels is a critical issue in tunnelling and in designing the lining and its sealing potential. This study was conducted to estimate the actual groundwater pressure on the lining of the tunnel in the Karaj–Tehran water conveyance tunnel. High water pressure (over 400 m) was estimated from the water level in boreholes whereas low water pressure (5 m) was measured at piezometers installed in the tunnel. In this regard, estimation of the groundwater head based on the water inflow into the tunnel is considered as a new approach. Water head and permeability, as fundamental parameters for the estimation of water inflow into the tunnel, are estimated through analytical methods and experimental studies prior to the tunnel boring. Because the actual data for water inflow into the tunnel are available, the water head can be estimated based on the volume of infiltrated water. Using these techniques, the water pressure along the tunnel axis is estimated to be between 5 and 170 m in different zones. This method is recommended for water head estimation in other tunnels with similar geological properties.