Kalmthout is a municipality in the Belgian province of Antwerp. The municipality comprises the villages of Kalmthout, Achterbroek, Heideā¦Kalmthout is a municipality in the Belgian province of Antwerp. The municipality comprises the villages of Kalmthout, Achterbroek, Heide, and Nieuwmoer. In 2021, Kalmthout had a population of 19,020. The total area is 59.45 square kilometres.