The free energy difference between the E and Z conformers of formic acid in the gas phase were calculated at the HF/6-31G(d), B3LYP/6-31G(d), and MP2/6-31G(d) levels, respectively. Free energy differences between the two structures in the gas phase were obtained from frequency calculations. The solvation free energies of the two conformers were obtained from the continuum CPCM model. The computed equilibrium constants in different solutions were compared to the latest experimental literature values.