725 km of hair: a rough lifetime estimate

An individual scalp hair does not grow for a whole lifetime: its active growth phase usually lasts years, after which it sheds and the cycle starts again. Adding the length produced by all follicles over many cycles can yield hundreds of kilometers. For example, estimates of about 100,000 scalp hairs and a growth rate around 0.35 mm per day give roughly 900 km over 70 years in a simplified calculation. The result varies with age, follicle count, growth phases, and lifespan. Thus 725 km is a possible popular approximation, not a measured population average.