From Daniel Engber’s “How Much of Me Is Burned?” (Slate: 11 July 2006):
In the 1950s, doctors developed an easy way to estimate the ratio of the area of a patient’s burns to the total area of his skin. The system works by assigning standard percentages to major body parts. (Most of these happen to be multiples of nine.) The skin on each arm, for example, covers 9 percent of a patient’s total surface area. Each leg comprises 18 percent, as do the front and back of the torso. The head and neck together make up another 9 percent, and the last bit (or 1 percent) covers the genitalia and perineum. This breakdown makes it easy for doctors to estimate the size of a burn in relation to a body—a burn that covered half the arm would add 4 or 5 percent to the total figure. …
Another method uses the size of a patient’s palm as a reference. As a general rule, the skin on the palm of your hand comprises 0.5 percent of your total surface area. (For children, it’s 1 percent.) A doctor can check the size of a patient’s hand and compare it with the size of a burn to make a quick guess about the percentage.