Deuterium oxide dilutions have long been the accepted means for collecting body condition measures on phocids. Blubber depth has become more frequently used as an indicator of health. This project represents the first in-depth examination of blubber depth measured via ultrasound as an indicator of body condition in a single species. We concurrently measured blubber depth via imaging ultrasound at 8 locations, fat and protein mass via deuterium oxide dilutions, morphometrics, and mass from wild, captive, and rehabilitating harbor seals (Phoca vitulina; n = 89). Seals ranged from premature to adult. By using body mass and blubber depth measurements we are able to predict fat and lean mass in an independent set of 187 animals (adjusted R² = 0.77 and 0.96, respectively). Although most of the ultrasound sites produced strong correlates to fat mass, an additive model incorporating body mass, 1 lateral site (L2), and 1 dorsal site (D4) provided the tightest fit.