The 100 per day number is based on the fact that most people have around 100,000 hairs, and at any given time 10% of them are in telogen (which is perfectly normal) and the telogen phase lasts around 3 months. 100,000 times 0.1 (10%) = 10,000 Ã· 90 (3 months) = 111. I also believe the notion that losing 100 hairs a day being normal due to personal experience. Back before I had any kind of hair loss I used to lose a noticeable amount of hair. I have no doubt at all that I used to lose that much, and maybe even more on some days. Also, a few days ago I was running my fingers through my boyfriend's hair, and in 5 minutes I counted 70 hairs. He has no hair loss whatsoever, in fact, he has the thickest head of hair ever, it seriously looks like a white afro. The following is how one of my doctors explained the hair cycle, shedding and hair loss to me: If a person is losing around 100 hairs a day, and they're experiencing visible/progressive hair loss, it's probably not due to excessive shedding. It could be because the hair lost is not being replaced as it should be. This is most often due to Androgenetic Alopecia. The only exception would be if a person doesn't have 100,000 hairs to start with. In that case losing around 100 hairs a day may make a difference after a while. This explanation makes sense to me.