The average human scalp has anywhere from 120,000 to 180,000 hairs.

The average hair grows at a rate of about 1/2 inch per month.

The average person's hair grows 6 inches per year.

Hair has an average growth cycle of 4-6 years.

Most people can grow their hair between 24 and 36 inches.

Some people have a very short hair growth cycle, only 1 & 1/2 years,
this means their hair will grow to a maximum length of nine inches.

Ten years seems to be the maximum growth cycle.  This accounts for people with extremely long hair (ex: Crystal Gayle), reaching 60 inches.

Each hair is formed and grows in it's individual follicle.

The hair follicle begins as a soft protein of specialized cells which become the cuticle, cortex, medulla and matrix of the hair.  As the assembled hair moves toward the surface it becomes hardened and keratinized. 

Hair outside the follicle is a dead protein keratin structure.

The growth period of the hair is called the Anagen phase.

When the hair stops growing it's in the Telogen phase.

During the Telogen phase the follicle withers and a bushy mass forms at the base of the hair shaft.

The hair is anchored in the scalp until after about 3 months when the growth cycle begins again.

When the cycle begins again a new follicle forms and as the new hair grows out it pushes the old hair out.