The use of a precursor simulation of fully developed turbulent channel flow for the generation of turbulent boundary layer (TBL) inflow data is investigated. Based on the desired properties of the TBL, a complete procedure is described for how to specify the precursor simulation. The key feature of the specification is to match the momentum thickness of the precursor to that of the inflow TBL. The inflow data is then constructed from time- and space-dependent flow data in a cross-plane of the precursor. The proposed procedure removes the need to rescale the flow data and thus violate the governing equations, as is common practice in other state-of-the-art inflow generation methods for TBLs. The adaption length of the generated TBL is investigated using wall-resolved large-eddy simulation (WRLES) for a zero-pressure gradient (ZPG-) TBL, with a momentum thickness Reynolds number in the interval 830–2 400. The results are compared with a solution obtained using a standard rescaling procedure for the inflow data. The adaption length is shown to be similar for the two methods. Practical differences and advantages of the proposed of method, as compared to other inflow generation techniques, are assessed. These involve the role of the auxiliary simulation, channel flow in the present case, in the overall computational procedure, as well as data handling, initial transients and adaption lengths.