Large eddy simulation, where large coherent structures are explicitly computed whereas the effects of smaller ones are modeled, appears as a very promising tool to numerically describe turbulent combustion. However, this approach requires the modeling of the unresolved reaction rate (i.e., reaction rate at scales unresolved during the simulation). In this paper, a similarity model, where the unresolved scales are assumed to behave like scales slightly larger than the cutoff scale, is proposed to estimate the subgrid-scale flame surface density. The model parameter is modeled assuming that the flame front behaves like a fractal surface at scales larger than a cutoff scale of the order of the laminar flame thickness. This model is tested against experimental data obtained using planar laser-induced fluorescence on the OH radical in a turbulent premised propane/air flame. Promising results are found: the similarity model reproduces correctly the locations of the unresolved reaction rate, whereas the fractal scaling analysis provides a very good estimate of the model parameter.