Food waste at the household level accounts for a significant share of total food waste in developed economies, according to the Food and Agriculture Organization. Studies have shown that this share varies between 0.3 kg to 4.5 kg per person per week, depending on the definitions and methodologies applied. In Italy, quantities, behaviors, and attitudes regarding food waste have been solely explored through the use of questionnaires, typically leading to discrepant values of food waste. In this study, we estimate and analyse the determinants of food waste over a 388 units’ panel spread over the national territory, through a diary and questionnaire study. Moreover, by comparing food waste value that was declared in questionnaires and reported in diaries, we confirm that the awareness of food waste quantities is heavily biased. The results confirm that the average food waste value is significantly higher when gathered through diaries, while questionnaires are able to catch less than one-third of food waste determinants.