In this work, a novel Weibull regression model, based on a former S N model developed by the authors, is proposed for the probabilistic definition of the N field as an alternative to the conventional Basquin-Coffin-Manson approach. The model arises from sound statistical and physical assumptions and, integrating the elastic and the plastic local strain, provides an analytical probabilistic definition of the whole strain-life field as quantile curves both in the low cycle and high cycle fatigue regions. The five parameters of the model can be estimated using different well established methods proposed in the fatigue literature, in particular, the maximum likelihood and the two-stage methods providing adequate estimates based on the set of experiments. Runouts, as a frequent case in fatigue testing, can be also dealt with. The model can be applied for probabilistic lifetime prediction using damage accumulation. Finally, the proposed N model is also used to derive the corresponding stress life curves, i.e. the probabilistic S N field that in this case presents the typical change in curvature in the low-cycle fatigue region.