Burn Area Calculator (Rule of Nines)

The Rule of Nines is a quick clinical tool for estimating the total body surface area (TBSA) involved in a burn. It divides the adult body into regions that are each approximately 9% of the total skin surface, or multiples of 9.

Head and neck: 9%. Each arm: 9% (18% combined). Anterior trunk (chest + abdomen): 18%. Posterior trunk (upper and lower back): 18%. Each leg: 18% (36% combined). Perineum: 1%. Total = 100%.

For each region, you enter what fraction of that region is burned, from 0 (none) to 100 (the entire region). The TBSA contribution from each region is: region weight * (fraction burned / 100). Add all contributions together for the total TBSA burned.

This calculator is for adults. Children have proportionally larger heads and smaller legs, so the Lund and Browder chart is used for pediatric burns. It adjusts each region by age group: an infant’s head is about 18% rather than 9%, and each leg about 14% rather than 18%.

Burn severity is classified by TBSA in adults: Minor: less than 10% TBSA (second-degree) or any small, isolated first-degree burn. Manageable outpatient. Moderate: 10-20% TBSA or burns involving face, hands, feet, or genitalia. Typically requires hospital admission. Major: over 20% TBSA, or any inhalation injury, or burns with circumferential involvement of a limb. Burn center referral is standard.

TBSA from first-degree burns (superficial redness only, like mild sunburn) is traditionally excluded from the calculation because these heal quickly without significant fluid loss.

The Rule of Nines is a rapid estimate, not a precise measurement. It takes about 30 seconds at the bedside, which matters in trauma care. For a more accurate calculation, the Lund and Browder chart or 3D scanning tools are used when precision is critical for fluid resuscitation volumes.

TBSA is not the whole triage decision

Severity is not a pure function of the percentage, which is why the calculator flags region involvement separately from the number. Face, hands, feet, genitalia and major joints are functionally critical: a 4% burn across both hands is a hand surgeon’s problem, not an outpatient dressing. Circumferential burns of a limb or the chest can cut off circulation or restrict breathing as the tissue swells, and may need escharotomy within hours. Inhalation injury changes everything and is easy to miss early, since the airway can look fine for the first hour and then close. Soot around the nose or mouth, singed nasal hair, a hoarse voice, or a burn sustained in an enclosed space are the signs that matter.

What the number is actually for: fluid

The main reason anyone computes TBSA in the first hour is to work out fluid resuscitation. The Parkland formula gives 4 mL × body weight in kg × %TBSA of lactated Ringer’s over the first 24 hours from the time of injury, not from arrival, with half of that in the first 8 hours and the rest over the following 16. The American Burn Association has since moved toward starting at 2 mL/kg/%TBSA for adults with flame and scald burns and titrating to urine output of 0.5 mL/kg/hr, because the original 4 mL figure was found to over-resuscitate a lot of patients. Both are starting points that get adjusted hourly against how the patient actually responds. Formal resuscitation is generally reserved for adults above 20% TBSA and children above 10%.