Vacancy and hydrogen concentrations in Al were determined by first-principles calculations and statistical-mechanics modeling, as functions of temperature and hydrogen chemical potential μH. Formation energies of Al vacancies, H interstitials, and H-Al vacancy complexes were obtained from first-principles calculations. The statistical-mechanics model incorporated these energies and included configurational entropy contributions through the grand canonical ensemble. We found that the hydrogen chemical potential under different chemical environments plays an important role in determining the relative equilibrium defect concentrations in the Al-H system. Estimates of the hydrogen chemical potential during hydrogen charging were obtained experimentally. At comparable the calculated concentrations are consistent with these values, along with previously reported measurements of hydrogen concentration.