Ethnicity influences average baseline growth rate. Asian hair typically grows fastest, African-type hair slowest on a straight-length basis.
Hair growth rate slows gradually with age, especially after 50.
Hormonal differences can affect growth rate slightly.
Curly and coily hair shrinks when dry, so the apparent length gain per inch of growth is less than for straight hair.
Hair grows slightly faster in warmer months due to increased scalp circulation.
Adequate protein, iron, zinc, and biotin support keratin production. Deficiencies can slow growth or trigger shedding.
Chronic stress can trigger telogen effluvium, pushing more follicles into the resting phase and slowing apparent growth.
Damage increases breakage, reducing the effective length you retain per month of growth.