We consider a domain Ω⊂Rd equipped with a nonnegative weight w and are concerned with the question whether a Poincaré inequality holds on Ω, i.e., if there exists a finite constant C independent of f such that
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It turns out that it is essentially sufficient that on all superlevel sets of w there hold Poincaré inequalities w.r.t. the constant weight 1 and that the corresponding Poincaré constants satisfy an integrability condition. Furthermore we provide an explicit bound of the constant C in the weighted inequality (1) in terms of the Poincaré constants of the superlevel sets. A similar statement holds true in the more general asymmetric case where we allow for certain weights ρ different from w on the right hand side of (1).