Body and feather mass data are important in avian studies and are required for determining things such as body condition and energetic carrying capacity. There are 12 subspecies of Common Moorhens (Gallinula chloropus), six continental and six island subspecies, of which two are endangered. Body mass data for multiple individuals are available for only three subspecies, and feather mass data have been reported for only one individual. Body mass (n = 82) and feather mass (n = 2) for adults and body mass for three subadult age classes (n = 27) are provided for the Hawaiian subspecies of Common Moorhen (G. c. sandvicensis). Other body size measurements, including tarsus length, shield-bill length, shield width, and wing cord length also are presented. Adult Hawaiian Moorhen body mass averaged 350.7 g (±50.0 SD; range, 232–522; 95% CI, 339.8–361.6), and young birds appear to develop like young of G. c. chloropus and other Rallidae. Based on published data, G. c. sandvicensis is heavier than G. c. guami, female G. c. chloropus, and G. c. meridionalis; lighter than G. c. garmani and males of G. c. cachinnans; and similar in mass to G. c. cachinnans females, males of G. c. chloropus, and G. c. orientalis. There do not appear to be systematic differences in body mass between mainland (data for four subspecies) and island subspecies (data for three subspecies). Total mass of all feathers for two males was 16.2 and 12.1 g, which made up 3.1% and 3.8%, respectively, of their total body mass.