The true available potential energy (APE) of an isolated feature (e.g. a warm‐core ring) in a basin is the difference in the potential plus internal energy between the observed and reference state. The reference state is that in which the same fluid is redistributed adiabatically to a stably stratified state and the isopycnals are on constant geopotential surfaces. For an isolated feature in an infinitely wide basin, it would seem reasonable to assume that the far‐field density structure is a good approximation of the reference state. This approximation to the reference state will be examined in this paper. Four different methods of calculating the available potential of an isolated feature are compared. These are (1) the gravitational available potential energy using the reference state (APEG), (2) the gravitational available potential energy using the far‐field density structure as an approximation for the reference state, (3) the Boussinesq available potential energy using the reference state (APEB), and (4) the Boussinesq available potential energy using the farfield density structure instead of the reference state. The gravitational available potential energy using the reference state (APEG) is the true available potential energy for an incompressible fluid. Therefore the different approximations of the available potential energy can be compared to the APEGof an isolated feature in an infinitely wide basin. It is found that the gravitational available potential energy using the far‐field density structure has an offset which depends on the density structure of both the feature and far field. Therefore the APEGusing the far‐field density structure is not a good method for calculating the APEGof an isolated feature in an infinitely wide basin, and the reference state must be calculated. We examine the limitations on the Boussinesq approximation for the APE. It is found that the isopycnal displacement from its reference state must be smaller than a length scale based on the reference state buoyancy frequencyNr2. Also, the maximum separation of isopycnals in the observed state must be less than twice the average separation of the isopycnals. With these conditions satisfied, the APEBapproximates the APEGof an isolated feature. The Boussinesq APE using the far‐field density structure instead of the reference state gives the asymptotic limit of the APEGof an isolated feature in an infinitely wide basin. A simple stratified model of an isolated feature is used to demonstrate the difference between these different available potential energy estimates. Finally, the APE estimates using some data from a survey of a Mediterranean salt lens a