This study investigated the accuracy of phantom volume and length measurements and the reliability of in vivo Achilles tendon (AT) volume, length and cross-sectional area measurements obtained using freehand 3-D ultrasound. Participants (n = 13) were scanned on consecutive days under active and passive loading conditions. In vivo AT length was evaluated using a two-point method and an approach that accounted for AT curvature (centroid method). Three-dimensional ultrasound provided accurate measures of phantom volume and length (mean difference = 0.05 mL and 0.2 mm, respectively) and reliable in vivo measures of AT volume, length and average cross-sectional area, with all intra-class correlations coefficients greater than 0.98. The mean minimally detectable changes for in vivo AT volume, two-point length and centroid length were 0.2 mL, 1.5 mm and 2.0 mm, respectively. Two-point AT length underestimated centroid AT length by 0.7 mm, suggesting that the effect of curvature on in vivo AT length is negligible.