An optimal botulinum toxin dose may be related to the volume of the targeted muscle. We investigated the suitability of using ultrasound and anthropometry to estimate gastrocnemius and soleus muscle volume. Gastrocnemius and soleus muscle thickness was measured in 11 cadaveric human legs, using ultrasound. Lower leg length was tape-measured. Muscle volume was determined by water displacement of the dissected muscles. Simple and multiple regression analyses, using muscle thickness and lower leg length as independent variables, were performed to establish muscle volume prediction equations from the muscle volume measured by water displacement. Validating the equations based on simple regression analyses resulted in a correlation (r²) of 0.373 and 0.518 (P < 0.047), and a standard error of the estimate of 24.4 cm³ (11.7% of the measured mean muscle volume) and 33.2 cm³ (12.7%) for the gastrocnemius and soleus muscles, respectively. The corresponding values for the multiple regression analyses were an r² of 0.497 and 0.650 (P < 0.017), and a standard error of the estimate of 21.9 cm³ (10.5%) and 28.4 cm³ (10.8%) for the gastrocnemius and soleus muscles, respectively. It seems possible to estimate the volume of individual plantar flexor muscles using ultrasound and anthropometry. This possibility should be investigated further in living humans.