The upward trend in the residential sector of energy use has significant consequences in terms of environmental impacts. Determining the carbon metric (CM), as part of the whole carbon footprint of a building, contributes to quantify the carbon emissions related to the building's use stage. Although many carbon footprint calculators exist in other sectors, none has been specifically designed and applicable to the building one. However, ISO 16745 provides guidelines for calculating and reporting the CM of existing buildings in operation. In this context, this work sets a methodology to measure the CM of existing households' use stage, based on ISO 16745 and split into three stages. The implementation of the methodology to a case study proved its applicability since it enabled the data collection task through the designed survey, and allowed the energy carriers and end-uses be disaggregated, quantified and clearly reported for user's knowledge. The study outlined that calculating the CM and, more specifically, reporting and making the results publicly available, help raise users' awareness about reducing greenhouse gas-related emissions, and provide new ideas for monitoring, benchmarking and proposing policies at individual member state and EU levels.