Objective The aim of the study was to explore an individualized sonographic muscle thickness ratio and its cutoff values in the diagnosis of sarcopenia. Design A total of 326 community-dwelling adults were included in this cross-sectional study. Total skeletal muscle mass was evaluated by bioelectrical impedance analysis, and nine-site muscle thickness measurements using ultrasound. Isometric handgrip and knee extension strengths were assessed. Physical performance was evaluated by usual Gait Speed, Chair Stand Test, and Timed Up and Go Test. Results Because the anterior thigh muscle thickness was the most significantly decreasing measurement with aging and the most significantly related value with body mass and height; sonographic thigh adjustment ratio was calculated by dividing it with body mass index. Using the two standard deviation values of our healthy young adults, sonographic thigh adjustment ratio cutoff values were found as 1.4 and 1.0 for male and female subjects, respectively. Sonographic thigh adjustment ratio values were negatively correlated with Chair Stand Test and Timed Up and Go Test in both sexes (allP< 0.05) and positively correlated with gait speed in female subjects and knee extension strength in male subjects (bothP< 0.05). Conclusions Our results imply that regional (rather than total) muscle mass measurements should be taken into consideration for the diagnosis of sarcopenia.