We compute the magnetic flux and helicity of an interplanetary flux tube observed by the spacecraft Wind on 24--25 October 1995. We investigate how model-dependent are the results by determining the flux-tube orientation using two different methods (minimum variance and a simultaneous fit), and three different models: a linear force-free field, a uniformly twisted field, and a nonforce-free field with constant current. We have fitted the set of free parameters for the six cases and have found that the two force-free models fit the data with very similar quality for both methods. Then, both the comparable computed parameters and global quantities, magnetic flux and helicity per unit length, agree to within 10% for the two force-free models. These results imply that the magnetic flux and helicity of the tube are well-determined quantities, nearly independent of the model used, provided that the fit to the data is good enough.