The European parliament has, during recent years, continuously updated their energy efficient requirements and directives underlining how much energy a building may use. Since the building sector accounts for such a large amount of the total energy use in Sweden, it’s important to make it more efficient. During a building’s planning phase, the energy use must be calculated in order to see whether the building will meet the building codes set by the Swedish National Board of Housing, Building and Planning. Unfortunately, it is very common that the actual use differs from the calculated, because of uncertainties in the input data. In this master thesis, input data that is used in energy calculations for apartment buildings has been studied. This has been performed thoroughly through a literature study and via computer simulations in which an apartment building is simulated using IDA ICE. Lastly, a sensitivity analysis was performed where the effect from variations in the input data was compiled. The results show that variations in hot-water use, indoor temperature and hot-water circulation losses also assumptions about thermal bridges and buildings mechanical systems can have significant impact on a building’s energy use. For new smaller apartment buildings the energy loss from hot-water circulation can amount to much higher than what is recommended as input data. Therefore, it is important to use as little standardized input data as possible to reduce the uncertainties.