Although the deltoid flap is widely described as a thin flap, no studies have reported on the measurement of its thickness. Thus, this study aimed to measure deltoid flap thickness at different points using brightness-mode ultrasonography and report our initial clinical experience. This study included 36 adults (26 males and 10 females; mean age: 34 years) with 72 healthy deltoid regions. Ultrasonography was employed to measure flap thickness at eight designated positions. The ultrasonography results were also applied clinically in 30 patients to evaluate its compatibility. The mean deltoid flap thickness was 5.14 ± 0.81 mm, with the thickest point being the emerging point of the flap pedicle. The subcutaneous fat gradually thinned toward the shoulder. No significant differences in the subcutaneous fat layer thickness were observed between the right and left sides, different ages, or the two sexes. Body mass index was the most critical factor related to flap thickness (p < 0.001). All deltoid free flaps were successfully transferred, resulting in good or excellent final clinical outcomes. The ultrasonography results suggest harvesting the deltoid flap upward toward the shoulder area and across the acromion from the emerging position of the flap pedicle to optimize flap thinness. The results showed that besides the flaps that have been clinically well established, the deltoid flap should be considered a valuable alternative for reconstructing limb soft-tissue defects, particularly where thin flaps are required and favorable aesthetic results are crucial.