Diagrams are developed for the correction of heating degree-days of a locality, which are quite often available at one base temperature only (e.g., 15.5°C or 18°C or 65°F), to the desired base temperatures that concern space heating applications (e.g., in the range of 14°C–20°C). The diagrams are based on the assumption of a simplified annual distribution of the ambient temperature values of the locality. This assumption—and consequently the validity of the proposed diagrams—is further examined according to detailed temperature data from many countries around the world, and found to be satisfactorily accurate.