This study investigates which position (grip span) on the standard grip dynamometer results in maximum grip strength. Our null hypotheses included (1) no optimal grip span exists for measuring grip strength and (2) optimal grip span is unrelated to hand size. We also intended to derive a simple mathematical algorithm to adapt grip span to hand size. Seventy healthy subjects (40 women/30 men; mean age, 40 years; range; 20-80 years) free of upper-limb lesions were evaluated. Each hand was randomly tested on 10 occasions using 5 different grip spans. Our findings showed that (1) optimal grip span was identified for both genders and (2) hand size and optimal grip span correlated in women but not in men. When measuring handgrip strength in women, hand size must be taken into consideration. We provide a mathematical equation (y = x/5 + 1.5 cm) to adapt optimal grip span (y) to hand size (x) in women. In adult men, optimal grip span can be set at a fixed value (5.5 cm).