People with limb loss may misjudge weight-related health when not adjusting body mass index (BMI) for amputation level. This cross-sectional, community-based study compared BMI and amputation-adjusted BMI (A-BMI) and evaluated relationships among BMI categories, function, and dieting. Subjects provided self-reported demographic, functional, and medical/prosthetic data including height and weight and completed performance-based balance and gait measures. A Web-based A-BMI calculator adjusted for amputation levels. Results for 294 subjects from 11 states (68.4% men; 76.5% white; average age, 55.6 [15.1] y) were reported, with vascular (49.7%) and unilateral transtibial (40.8%) amputations as the most common. Body mass index and A-BMI were closely correlated (Pearson r = 0.99), but a BMI of 28.6 (6.7) was less than an A-BMI of 30.3 (6.2) (t test, P < 0.001). Agreement among Centers for Disease Control BMI categories was moderate (κ = 0.48); 39.7% of BMI categories were underestimated without adjusting for amputations. Functional measures did not differ among BMI/A-BMI categories (P > 0.05). A larger than random proportion categorized as overweight by BMI dieted (χ2, P < 0.05), and people categorized as overweight by A-BMI did not (38.3% classified as normal BMI). Functional abilities did not differ among BMI categories (Kruskal-Wallis and analysis of variance, P > 0.05). People with limb loss using BMI may underestimate weight-related health; a Web-based A-BMI calculator may help monitor weight to make dieting decisions.