A piezometric head method is proposed to estimate the impermeable layer depth used in drain spacing formulae. The method was tested and compared with soil texture changes and the US Bureau of Reclamation (USBR, 1978) criterion of impermeable layer being one with hydraulic conductivity one tenth to one fifth that of the weighted hydraulic conductivity of the upper layers. The proposed method, which works for below the water table conditions and does not require hydraulic conductivity measurements, gave approximately the same results as the USBR method. The soil texture changes method was erratic and not reliable.