The BMI measured in the FCS 2014 served as golden standard. Overall, the self-reported BMI was underestimated in the FCS 2014. The underestimation with one BMI-unit is within the range of other studies [16, 24]. Hence, the prevalence of obesity was underestimated when based on self-reported BMI which is in accordance with many other studies as well [3, 13, 17–19]. The misclassification frequency is an appropriate way to assess the accuracy of self-reported BMI .