The tool presented was developed with the purpose of helping architects and engineers in the process of selecting the optimal solution of windows for dwellings. It can be used during the design phase of new dwellings or for the renovation of existing ones. Built in Microsoft Office Excel 2007 and Visual Basic for Applications (VBA), the tool aims to be user-friendly and based on simple input data. At the same time, it is adapted to different expertise levels: for example the inexperienced user has the option of using pre-defined solutions and default suggestions, while the experienced user can have a very high level of flexibility. The method/tool organizes the process of selecting windows in four different stages named as Step1, Step 2, Step 3 and Step 4. In Step 1, the user can evaluate and compare the energy performance of different individual windows based on the knowledge of their geometries and components (glazing and frame). This first evaluation is based on the concept of the net energy gain defined in Nielsen T. R. et. al. (2001). In Step2, picking from the windows previously characterized, the user can define a complete set of windows for a specific dwelling. For each window the user must specify orientation, obstructions from horizon, overhangs and fins and solar shading device. The user can create different sets (scenarios) of windows for the dwelling. Depending on the flexibility of each particular design case, the scenarios may be different regarding several aspects (ex. windows components, windows geometry or windows orientations). On a seasonal basis (winter/summer), the energy consumption of the windows used in the dwelling as well as the energy consumption of the dwelling are calculated for each scenario. The calculation is made taking into account the gain and loss utilization factors for heating and cooling, respectively, according to CEN(2008, ISO 13790). The basis of Step 3 is the “simple hourly method” defined in CEN(2008, ISO 13790. In this stage the indoor temperature and the heating/cooling energy demand are calculated on an hourly basis for a critical room of the dwelling. The main goal of this stage is to verify whether or not the windows defined for each scenario also allow fulfilling the indoor comfort requirements defined in CEN( 2007, EN 15251). The Step 4 consists of an economic evaluation for the scenarios of windows previously defined. In this stage, it is possible to calculate the cost of conserved energy when using the selected windows solutions, in comparison to a reference solution. Based on the overview of the analyses made during the four steps, the user is, at this stage, able to select the windows solutions with the optimal performance in the actual dwelling. Furthermore, the user is not obligated to follow the four steps. The user may only use Step 1 to have a very quick idea of the energy performance of different individual windows with regard to geome-try and components. Or the user may use only Step 2 in order to perform a seasonal calculation knowing previously the U-value and g-value of the windows that he wants to use. Step 3 and Step 4 are independent from each other but require Step 2 to be previously performed.