This is not a simple question as there are many variables that can alter the number of hairs required to cover an area with a natural looking density.
Hairs grow in natural groupings called follicular units; there are generally one to four hairs per follicular unit and dependent on the area to be treated will be strategically placed in the recipient area to give the best cosmetic impact.
Hair line work requires the use of single hair follicular units directly on the hair line to give a soft, natural look, not a hard wall of hair, behind the hair line the multiple hair units are placed to help gain the fullness of the result. Hair characteristics play an important role in the number of hair required, fine, wispy hair gives little coverage whereas coarse wavy hair gives a much greater illusion of thickness. The same with hair to scalp colour contrast, low contrast, a better look.
Hair density in the donor and recipient areas is generally measured per cm²; the density around the sides and back varies, with a slightly lower hair density around the sides and a higher density around the back.
With a hair transplant the result works by illusion of density, blocking the light reflection off the head, the more the reflection can be blocked the greater the look of thickness is achieved. Single hair follicular units (FU´s) require a greater number to be placed to create this illusion of thickness, dependent on the hair characteristics possibly from 40-70 FU´s per cm².
Multiple hair units can reduce the density placed due to more hairs per follicular unit; the density placed could be 30% less but the same % more hairs in that area.
Placing a high density is a skill, creating the slits for the graft to be placed in such a small area needs precision tools; at Hattingen Hair we use custom-made blades, to alter the size of the slit dependent on the graft size we place. Being able to vary the slit size also allows for a greater density and speedier healing at high density; a skill required on small or large procedures, but especially larger procedures having to cover a wider surface area.
Hair density is a vital component to a successful hair transplant, it is important that whatever the area is that needs to be treated it is completed in one pass with the required density. It is not ideal to have to go back into the same area
The exception to this is when rebuilding a higher Norwood scale hair loss sufferer and total restoration in one procedure is not possible. Then the density placed will drop going back over the head to not leave a flat hard line.