Objective : To assess the validity and reliability of girth measurement for calculating the residual limb volume. Design : Comparison of girth measurement with gold standard in calculating residual limb is volume. Setting : King Chulalongkorn Memorial Hospital and Sirindhorn National Medical Rehabilitation Center. Subjects : 51 below-knee amputees Methods : Residual limb volumes of each subject were measured by water displacement method with volumeter and girth measurement method by measured segmental circumferences with a tape measure twice for each method. Each segment volume was calculated by frustum and cylinder formulas of girth measurement method and summed them up to be the calculated residual limb volume. The stump volumes were measured again 4 weeks later in subjects who had their limbs amputated less than 6.5 months. Mean difference and 95% confidence interval (CI) between volume of water displacement and girth measurement, 1st and 2nd calculated volume of girth measurement, and changing volume of water displacement and girth measurement were calculated by simple calculation. Results : The difference of mean volume (95% CI) of water displacement and girth measurement, frustum and cylinder formulas were -14.03 cm[superscript 3] (-27.23,-0.84) and -13.52 cm[superscript 3] (-26.70, -0.35), respectively.The mean difference of calculated volume (95% CI) of the 1sand 2nd measurement of frustum and cylinder formulas were -4.23 cm[superscript 3] (-8.09, -0.37) and -4.2 cm[superscript 3] (-8.11,-0.40). The mean difference of changing volume (95% CI) of water displacement and frustum formula, and water displacement and cylinder formula were 3.81 cm[superscript 3] (-8.02,15.64) and 3.88 cm[superscript 3] (-7.99,15.74). Conclusion : The girth measurement, both frustum and cylinder methods were valid and reliable for calculating the residual limb volume in below knee amputees. But both of them might not valid enough to calculate the small changing volume.