Higher relative adiposity is independently associated with adverse health consequences. As such, accurate estimates of body composition in field settings are of utmost importance. The aim of this study was to examine the validity of the Body Adiposity Index (BAI) and Relative Fat Mass (RFM) as measures of total body adiposity. BAI and RFM were assessed in young adults (n = 188, 48.4% female, 21.8 ± 4.8 years, 24.4 ± 4.6 kg/m[superscript 2]). Adiposity was assessed using the 4C model (%Fat[subscript 4C]) and derived from underwater weighing for body density, dual-energy X-ray absorptiometry for bone mineral content, and bioimpedance spectroscopy for total body water. Bivariate correlations between BAI, RFM, and %Fat[subscript 4C] were assessed using Pearson's r. Differences between measures were assessed using a repeated-measures ANOVA. BAI and RFM were associated with %Fat[subscript 4C] (r = 0.668 and 0.827 respectively, both p < 0.001). BAI and RFM overestimated %Fat4C (+4.3 ± 6.1 versus +4.3 ± 4.6, respectively, p < 0.001). The large error limits the practical utility of these anthropometric indices.