What Is the Rule of Nines?

Medically Reviewed by Dan Brennan, MD on November 27, 2021
4 min read

The rule of nines is a tool used to estimate a burn's percentage of your total skin. It divides your body into sections by multiples of 9% each.

The sections in the rule of nines are:

  • Head: 9% 
  • Genitalia: 1%
  • Arm: 9%
  • Leg: 18% 
  • Torso: 36%

The body sections can divide in half. For example, the front side of one arm or your head is 4.5% of your total body surface area. The front and back of your torso are 18% each. 

These percentages are accurate for people over age 14.

The rule of nines gives an idea of how much of your total body's surface area a burn takes up. This informs treatments based on the size and intensity of the burn injury.

Emergency medical responders are some of the medical workers who use the rule of nines most. They quickly estimate the burn area to decide on treatments on the way to the hospital. 

The rule of nines applies to burns from all causes. The most common causes of burn injuries are:

Thermal.Thermal burns result from contact with a hot surface, object, or flames. The heat kills your skin cells. Common causes of thermal burns include:

  • Steam 
  • Boiling liquids
  • Hot metals 
  • Fires

Electrical. Electrical burns come from direct contact with an electrical current. Electrical currents are high amounts of energy that flow through wires. Accidents that cause electrical burns include:

  • Touching exposed wires
  • Using faulty machinery or electrical equipment
  • Touching a power line or pole
  • Touching an electrical device that water is touching, such as a hair dryer in the sink while it’s still plugged in
  • Lightning

Electrical burns can damage other areas besides your skin. Electric shock can weaken your muscles, impact your vision, or in severe cases stop your heart. 

Chemical. Chemical burns come from contact with harsh or dangerous chemicals. These chemicals can cause damage similar to thermal burns. Chemicals that can burn your skin include:

  • Drain cleaners
  • Wet cement
  • Bleach
  • Battery acid

Your skin has multiple layers between its surface and underlying tissues like muscle and fat. Burns have degrees based on how deep they reach into your skin.

First-degree. First-degree burns affect only the top layer of skin (called the epidermis). They can cause redness, skin dryness, and pain. First-degree burns usually heal with no scarring. 

The rule of nines usually isn’t needed in first-degree burn care.

Second-degree. Second-degree (or partial-thickness) burns reach the deeper layers of your skin (called the dermis). They often swell and blister. Second-degree burns can leave permanent scars.

Third-degree. Third-degree (or full-thickness) burns reach to the deepest part of your skin and parts below the skin, like hair roots and sweat glands. 

Fourth-degree burns reach underlying fat. Fifth-degree burns reach muscle. Sixth-degree burns reach bone.

Burn injuries can seriously damage your skin and other parts of your body. Your immune system’s response to a severe burn can lead to shock, heart failure, and organ damage.

Medical providers need to know a burn’s relative size using the rule of nines to decide on treatments. 

Burn treatments include:

Skin grafts. Skin grafting is a treatment that takes healthy skin from one part of your body and places it on an injured area. This helps protect the injury from infection. 

Determining a burn injury’s total body surface area using the rule of nines can help doctors plan a skin graft treatment.

Fluid replacement. Severe burns can cause your body to lose too much fluid, like blood, sweat, and water, inside your cells. This means you may need a fluid IV to keep enough water inside your body. 

A burn’s amount of total body surface area informs how much water should go in your IV line.

Human error. Burns can spread over your body so they are unevenly distributed across the rule of nines’ sections. A burn injury’s size, shape, and depth can make it hard to guess its percentage of total body surface area. 

Inaccurate guesses can cause doctors to use too much fluid or too little fluid in treatment. This can lead to kidney problems, liver damage, swelling, and other issues.

Different body sizes. The rule of nines doesn’t apply to children or patients who are obese. Babies and children under 14 have different rules for estimating a burn’s percentage of total body surface area. 

Many other rules for estimating a burn’s size can apply to patients who are obese. Based on their body shapes, different body parts have different percentages of total body surface area.