Use the Rule of Nines to Calculate a Sunburn Area

Overall, burn severity is a measurement of the depth of burning and the size of the burn. Measuring the size of a burn is difficult because every person is different in size, shape, and weight. It's impossible to simply choose what universal size of a burn is significant. A square foot of burned surface area is much worse to a person who weighs 130 pounds than it is to someone who weighs 200 pounds.

A woman rubbing a wet towel on her severe sunburn
Sara Lynn Paige/Getty Images

To account for inequities in size and shape, the burned surface area is calculated as a percentage of the total body area. Of course, we don't actually know how many square inches of skin covers any single person, but we do know about how much of our skin it takes to cover our arms and legs, for example.

The Rule of Nines

To approximate the percentage of burned surface area, the body has been divided into eleven sections:

  • Head
  • Right arm
  • Left arm
  • Chest
  • Abdomen
  • Upper back
  • Lower back
  • Right thigh
  • Left thigh
  • Right leg (below the knee)
  • Left leg (below the knee)

Each of these sections takes about nine percent of the body's skin to cover it. Added all together, these sections account for 99 percent. The genitals make up the last one percent of the total body surface area.

To apply the rule of nines, add up all the areas of the body that are burned deep enough to cause blisters or worse (2nd or 3rd-degree burns). For example, the entire left arm and the chest covered in blisters would be 18 percent. Partial areas are approximated. For example, the face is only the front half of the head and would be considered 4.5 percent.

Since kids are shaped so much differently than adults, there are adjustments made to the rule of nines, which of course ruins the point of making this tool the rule of nines. Indeed, so many variations exist for different age groups that it would be fruitless to go into them here.

The most important thing to remember about the rule of nines is that it is intended to be used in the field to quickly determine if patients need to go to a specialty burn center. Once the patient is in a burn center, more advanced techniques will be used to determine the exact burned surface area.

The total burned surface area is not the only thing that determines if a burn is critical or not. The degrees of the burn is also important to figure out. 

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By Rod Brouhard, EMT-P
Rod Brouhard is an emergency medical technician paramedic (EMT-P), journalist, educator, and advocate for emergency medical service providers and patients.