Determining body composition in gray seals (Halichoerus grypus) is important when studying their physiology and life history. In this study we investigated the predictability of total body fat (TBF) and protein (TBP) in postweaned gray seal pups from morphometric measurements, blubber thickness using ultrasonographs and bioelectrical impedance analysis (BIA). In postweaned pups, TBF (kg) could be estimated from girth measurements and sex (n= 45, r2= 0.878) using hydrogen isotope dilution methods as the reference. However, TBP could not be reliably estimated from morphometric data. TBF (kg) in yearlings was best predicted from mass (n= 6, r2= 0.776) and TBP (kg) from mass/length (r2= 0.949). Dorsal blubber thickness using B-mode ultrasound was also a significant predictor of TBF (kg) in postweaned pups (r2= 0.725) but BIA was not. Marked pups were recaptured during their first few months of life (n= 48) and body composition changes investigated. Animals lost mass and TBF after leaving the breeding beach, largely during the first 5–6 mo of life. Postweaned pups were ∼40% TBF and ∼13% TBP whereas yearlings were ∼12% TBF and ∼20% TBP. Pups that survived beyond 6 mo of age then regained mass as protein. Morphometric measurements are a useful field indication of body condition when isotope dilution is impractical.