The efficacy of cross-sectional fat areas in estimating total body fat was investigated in a sample of white American children and adults. Body density and total fat weight (kg) in the body were determined by hydrostatic weighing. Fat areas were calculated for the arm and calf using the appropriate limb circumferences and skinfolds measured at the triceps, biceps, and calf sites; also, a fat area was calculated using the average of triceps and biceps sites and arm circumference. Cross-sectional fat areas do not estimate body density (and percentage fat) any better than the corresponding skinfolds. In estimating weight of fat in the body, however, fat areas are systematically better estimators than corresponding skinfold thicknesses.