The hydraulic performance of thin-wall lay-flat polyethylene pipes with non-coaxial emitters were evaluated in this study. Two experiments were performed to measure the dimensional variation of commercially available lay-flat drip irrigation pipes. A model for estimating the corresponding local head loss was developed and validated using dimensional analysis. The model was then implemented in a finite element algorithm to analyse the hydraulic performance of this type of pipes. Finally, a design example was provided. The results revealed that, for a wall thickness of 0.13 and 0.20 mm, the pipe height and width tended to be equal above a relatively low pressure, and the pipe cross-section rapidly inflated from a quasi-rectangular form to circular. For thicker walls, the equivalent diameter (four times the hydraulic radius) increased continuously with increasing pressure head but it hardly reached its nominal diameter (16 mm). The proposed model indicated that the local head loss was affected by the equivalent diameter, emitter geometry, and flow velocity. The model had high accuracy in determining the local head loss. The estimated results were in close agreement with the measured data. According to the numerical simulation, the ratio of local head loss to total loss along a pipe was up to 74.71%. The design results, when using an earlier model, demonstrated a longer maximum lateral length, since the model did not consider further contraction of the flow caused by pipe deformation. However, the model proposed here could be used in the design of systems for thin-wall lay-flat polyethylene pipes.