How long hair can grow in three months varies from person to person and is related to age, nutritional status, environmental temperature and other factors, generally 3~4cm. 1. The growth rate of hair is affected by many factors, such as age, nutritional status and environmental temperature. Under normal circumstances, the length of hair growth in a day and night is about 0.3mm, and the length of hair growth in a month is about 1cm, all, the length of hair growth through three months, the increase in length is about 3~4cm. 2. The hair growth rate of the elderly is the slowest, followed by middle-aged people, children as well as teenagers have the fastest hair growth rate. The hair growth rate of people with excellent nutritional status is significantly better than that of people with malnutrition. In addition, hair growth is slower in the winter and faster in the summer when temperatures are higher. Due to the number of influencing factors, it is difficult to assess specific data. If there is a significant slowdown in hair growth, you can go to the hospital to clarify the cause and follow the doctor’s instructions.