I will discuss finite volume matrix elements of local operators in integrable quantum field theories and present a conjecture relating finite and infinite volume form factors in the most general setting, i.e. including the case of form factors containing disconnected pieces in the presence of non-diagonal scattering. Specializing to the sine-Gordon model, finite volume multi-soliton form factors are compared to numerical data coming from a numerical renormalization group calculation in the truncated conformal space. I find excellent agreement between the two approaches, thus verifying both bootstrap for multi-soliton form factors and the theory linking finite and infinite volume matrix elements. Evaluation of the nontrivial multi-soliton form factors is achieved by a newly developed regularization scheme.