A callus is caused by the buildup of hard, dead skin over an area of pressure or bony prominence. The hard skin is produced by special skin cells called keratinocytes, which are present throughout the entire epidermal layer of skin.
Calluses are the body's attempt at protecting itself from friction or pressure, but unfortunately, the extra skin becomes a painful problem in many cases. They are most common on the heel, the ball of foot, and the sides of the foot.
You may have a callus if you have a large area of thickened skin, especially on the heel, ball of foot, or sides of the foot.
Calluses on the feet are often caused by friction and pressure from footwear as well as abnormal patterns of walking or repetitive activities (such as in sports).