The skin is the major organ in the integumentary system, one of eleven major human organ systems comprising 78 distinct organs. Our flesh represents some seriously sophisticated, all-natural biotechnology for advanced animals. It has to be able to keep the good stuff coming in and the bad stuff out, which is no easy task.
The soft outer envelope of the skin evolved to cover vertebrates; it’s what we have instead of a stony exoskeleton. As the biggest organ for mammals in general, our skin is the heaviest organ and has the largest surface area.
In general, adults have a total surface area of 1.5 to 2 square-meters of skin, or about 22 square-feet. Most skin is an average of 2 to 3mm thick, about as thick as a piece of craft felt fabric. That’s just an average, though — across the body, skin thickness varies quite a bit depending on location. On areas like the upper back, coverage can be as thick as 5mm, while the thinnest eyelid flesh is around 0.05mm.
An adult’s dermal layers collectively weigh from six to eight pounds—about twice as hefty as either the brain or the liver. The liver weighs in at between three and four pounds, making it the second largest and heaviest organ in the human body. Keep in mind that your skin is an external organ, giving the liver the number one spot as far as internal organs are concerned. (Source)