Mathematical relationships relating the biomass to the surface area of Elodea canadensis Michx., Myriophyllum spicatum L., Nitellopsis obtusa (Desv.) J.Gr., Potamogeton lucens L., Potamogeton pectinatus L. and Potamogeton perfoliatus L. were developed. The methods leading to these relations are presented for plants from Lake Geneva (Switzerland). Results include three sets of data for each species: leaves, stems and whole plant. Differences were evidenced according to plant species; for a same biomass E. canadensis and M. spicatum offered the highest surface area, P. pectinatus and N. obtusa the lowest. For 1.0 g dry weight of whole plant, the surface area was 1255 cm2 for E. canadensis, 1205 cm2 for M. spicatum, 560 cm2 for N. obtusa, 653 cm2 for P. lucens, 500 cm2 for P. pectinatus, and 762 cm2 for P. perfoliatus. Plants with dissected morphology did not necessarily offer the largest surface area per unit biomass.