Free energy perturbation calculations have been applied to calculate the absolute binding free energies of K+ with macrocyclic ionophore 18-crown-6 in methanol and in water and its acyclic analogue, pentaglyme in methanol. For 18-crown-6, the free energies of binding with KC were evaluated as -1.2 kcal/mol in water and -5.2 kcal/mol in methanol, compared with experimental values of -2.9 kcal/mol in water and -8.4 kcal/mol in methanol. A simple force field model was used to estimate all available conformations and their relative energies for pentaglyme in solution. This allowed the estimation of the extra configurational free energy price pentaglyme has to pay in order to stay in the conformation optimal for binding K+. In contrast, 18-crown-6 pre-exists in this conformation in solution. The total calculated free energies of binding in methanol were evaluated as -5.2 kcal/mol for 18-crown-6/K+ and 0.5 kcal/mol for pentaglyme/K+, compared with experimental values of -8.4 and -3.0 kcal/mol, respectively. The difference in binding free energies between 18-crown-6 and pentaglyme, which is usually described as the macrocyclic effect, is -5.7 kcal/mol from the calculations and -5.3 kcal/mol from experiments. Both the configurational free energy and the binding interaction contribute to this difference.