The rule of nines is a method doctors and emergency medical providers use to easily calculate the treatment needs for a person who’s been burned.

It’s sometimes referred to as the Wallace rule of nines after Dr. Alexander Wallace, the surgeon who first published the method. The creation of this method is credited to Pulaski and Tennison.

A medical professional will do a visual examination to look for seriously burned areas and use the rule of nines to quickly add up what percentage of a person’s body is burned. While doctors will perform more thorough examinations for burn estimation, they can use the rule of nines to quickly assess a person and start recommending treatment centers and interventions to help a person.

The rule of nines is meant to be used for:

  • second-degree burns, also known as partial-thickness burns
  • third-degree burns, known as full-thickness burns

The rule of nines assigns a percentage that’s either nine or a multiple of nine to determine how much body surface area is damaged. For adults, the rule of nines is:

Body partPercentage
Arm (including the hand)9 percent each
Anterior trunk (front of the body)18 percent
Genitalia1 percent
Head and neck9 percent
Legs (including the feet)18 percent each
Posterior trunk (back of the body)18 percent

If a person’s injured due to a burn, a doctor may assess them quickly. For example, if they were burned on each hand and arm as well as the front trunk portion of the body, using the rule of nines, they’d estimate the burned area as 36 percent of a person’s body.

A medical provider can use calculations from the rule of nines in several ways. This includes the amount of fluid replacement and degree of care a person needs.

When a person experiences a second-degree burn or worse, the protective layer of skin is destroyed. As a result, they’ll lose a significant amount of body water. This makes providing fluids vital to helping a person maintain their total body water. According to the National Institutes of Health, burns that are greater than 20 to 25 percent of total body surface area require significant intravenous (IV) fluids. Doctors will also use the estimated body surface area burned to determine how much fluid to administer.

The rule of nines can also relay to a medical team receiving the patient how serious the injury is. Providers also know that burns that exceed 30 percent of a person’s body can be potentially fatal, according to the National Institutes of Health.

If a person has burns on 10 percent of their body surface area or greater, a specialized burn center should treat their wounds. Other circumstances where a burn center should treat the wounds include:

  • when the person is a child
  • when the burned areas involve key areas of the body, such as the hands, feet, genitalia, face, or major joints
  • chemical burns
  • electrical burns
  • the presence of third-degree burns

Another example of how a provider may use the rule of nines is to determine how much IV access is needed. If a person has 15 percent or more of their total body surface area burned, they’ll need at least one peripheral line to provide IV fluids. If a person’s body is burned 40 percent or more, they’ll need at least two IVs.

Doctors don’t typically use the same calculations in the rule of nines to children. This is because children tend to have different body proportions than adults do, including larger heads and smaller legs. For example, children tend to proportionally have a 20 percent larger head than adults, according to the National Institutes of Health. Infants also have 13 percent smaller legs than adults.

Therefore, there are a few adjustments for the rule of nines in children:

Body partPercent
Arm (including the hand)9 percent each
Anterior trunk (front of the body)18 percent
Head and neck18 percent
Legs (including the feet)14 percent each
Posterior trunk (back of the body)18 percent

Burns are a serious, painful injury that require immediate treatment and intervention. The rule of nines serves as a quick method of assessment for a medical provider to estimate the extent of a person’s injuries. If the person with burns is a child, the rule of nines should be adjusted due to differences in a child’s proportions.