More than 40% of adults aged 65 and older experience physical disabilities that affect their daily activities, impacting their independence and quality of life. A study investigated age-related changes in body composition using C57BL/6 mice to understand the link between lean mass, total mass, and age-related weakness. Researchers utilized the EchoMRI-500 body composition analyzer to measure lean mass and correlate it with total mass across various age groups and sexes. Findings indicate that both lean and total mass increase with age in both male and female mice. However, the lean-to-total mass ratio declines with age, mirroring the reduction in lean mass seen in humans. Male mice demonstrated greater muscularity than female mice in older age, with lean-to-total mass ratios of 0.525 for males and 0.203 for females. This study presents a straightforward method for estimating lean mass from total mass, which could be beneficial for researching age-related weakness.