The nonlinearity of the relationship between the extensive and the intensive variables of an ideal energy‐storing thermodynamic system in equilibrium is expressed in terms of the higher‐order moments of the extensive variable, and is thus shown to have both an upper and a lower bound. The bounds are proportional to the size of the system and for most practical systems the upper bound is more restrictive. As an illustration, the bound is applied to the nonlinearity of varactor diode capacitors, and the lower bound is shown to be far from the presently achieved nonlinearity.