According to wikipedia , the answer is about 1.25 cm/month (or 0.5 inch/month), however yesterday I found out a nicer way to express this speed :) Sheldon Cooper (The Big Bang Theory - Episode 5x18, link below) claims that is 4.6 yoctometers per femtosecond (so 1.2 cm per month or 0.47 inches per month). Interesting how you can use two very small scales (1 yoctometer = 10^-24 meter and 1 femtosecond = 10^-15 s) to express such an everyday-life-thing as the hair growth! On the scale of yoctometers, a proton is gigantic! Indeed a proton is 10^-15 meters so 1 billions of yoctometers! Femtoseconds are bit more close to our everyday common sense, for example the visible light waves oscillate on the femtosecond scale (for a wavelength of 580 nm the corresponding period is 1.93 fs). Concerning the length scales of the incredibly small and incredible big things, I recommend you these nice pages The Scale of the Universe The Scale of the Universe 2 PS By the way, here is the orig...