The study of the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional φ^4 models can be performed in several representations, and the amplitude-phase (AP) Madelung parametrization is the natural way to study the contribution of density fluctuations to non-universal quantities. We show how one can obtain a consistent phase diagram in the AP representation using the functional renormalization group scheme. Constructing the mapping between φ^4 and the XY models allows us to treat these models on equal footing. We estimate universal and non-universal quantities of the two models and find good agreement with available Monte Carlo results. The presented approach is flexible enough to treat parameter ranges of experimental relevance.