Background: Low skeletal muscle index (SMI) and sarcopenia adversely affect clinical outcomes in oncology patients. Study aims were to assess the agreement of bioelectrical impedance analysis (BIA), mid-arm muscle circumference (MAMC), and computed tomography (CT) at the third lumbar vertebra (L3), for the measurement of muscle mass and identification of sarcopenia, in patients with colorectal cancer (CRC). Method: A comparison study of low SMI and sarcopenia determined by BIA and MAMC, compared to CT. Sensitivity, specificity and area under the curve (AUC) were calculated. Results: CT scans were obtained for 100 participants. Low SMI was identified in 29%, 57%, and 20% of participants using CT at L3, BIA, and MAMC, respectively. For low muscle mass BIA showed 60% of participants were correctly classified (AUC 0.619, sensitivity 80%, specificity 52%, kappa 0.241, P = 0.009) and for MAMC, 73% of participants were correctly classified (AUC 0.625, sensitivity 38%, specificity 88%, kappa 0.286, P = 0.005). There were 14%, 31%, and 10% of participants identified as having sarcopenia from CT, BIA, and MAMC, respectively. Conclusions: Both BIA and MAMC show a poor level of agreement for measuring muscle mass compared to CT scans using L3 in patients with CRC.