A methodology to characterise the pressure losses in quasi-steady conditions (i.e. at full needle lift) of common rail diesel injectors was developed. The aim was to quantify the error when experimental results of nozzle internal flow are compared with computational fluid dynamics results, where pressure losses are usually neglected. The proposed methodology is based mainly on experimental tests that are complemented with some approximate calculations, based on the physics of the phenomenon, to take into account the effect of the needle deformation. The results obtained in the work lead to two important conclusions: on the one hand, that it is dangerous to extrapolate results relative to the injection (internal flow, spray atomization, spray penetration, etc.) and combustion processes from low permeability nozzles (e.g. single-hole nozzles) to high permeability nozzles (e.g. multi-hole nozzles), and, on the other hand, that the comparison of these results between experiments and computational fluid dynamics simulations should be carried out carefully, because the pressure losses in the injector can be high under certain conditions. Finally, people working on the study of the injection and/or combustion processes, through experiments or simulations, will find here some interesting information to better know the actual injection pressure to be used in their analysis and/or simulations.