A retrospective study evaluated the correlation between pectoral muscle cross-sectional area (PMA) and bone mineral density (BMD) in estrogen-deficient postmenopausal women. The study initially included 500 subjects from August 2023 to August 2024, leaving 338 after exclusion criteria. Participants were divided into groups with normal bone density, osteopenia, and osteoporosis. Age, BMI and PMA were strongly correlated with BMD, with PMA positively correlated with the lowest BMD (r = 0.448). There was good inter-rater agreement (ICC = 0.980, p < 0.05). Multiple linear regression after adjusting for age and BMI gave the formula: Lowest BMD = 0.858 – 0.005 × age + 0.006 × BMI + 0.005 × PMA. Reduced muscle mass increases the risk of osteoporosis, and simple measurements from a routine chest CT scan can provide information on BMD.