This question has been explored by German researchers who used two different hand rubs on volunteers to identify the minimum volume required to achieve clean hands. They also explored whether hand size influenced the amount of product required.
The study looked at eight male volunteers with big hands, and eight female volunteers with significantly smaller hands.
Females with smaller hands needed up to 3mls of product while males with larger hands needed up to 4 ml.
The authors conclude that the volume of hand rub used should not fall below 3 ml in order to achieve maximum benefit.