Energy metabolism has a linear relationship with body surface (BS). However, this surface area cannot be easily measured. Consequently, a simple formula including body mass (BM) and body length (BL) is proposed for the eel. The formula BS = BL0.53 × BM 0.49 × 3.983 gives BS calculated values which show very low variations compared to the measured values (0.16 ± 1.07%). The formula may be employed in the study of eel energetics and various aspects of applied aquaculture.