This paper introduces reachable power flow (ReachFlow), a formal method providing a provably over-approximated enclosure of the complete set of the power flow solutions that “can be reached” under various uncertainties. The novelty of ReachFlow lies in: (1) an ordinary differential equation (ODE) formulation which maps the iterative power flow solving process into a virtual dynamic; (2) a reachability analysis of the virtual ODE model which enclose all possible (infinite) power flow solutions under uncertainties in one calculation. Case studies verify the efficacy of ReachFlow as a formal method and, in particular, its capability of handling islanded droop-based microgrids under uncertainties.