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%.

When to call a burn center

The American Burn Association’s referral guidelines (2022) are the ones US emergency departments work from, and they are blunter than the old minor, moderate and major bands. Immediate consultation, with transfer considered, for:

• any full-thickness (third-degree) burn, of any size
• partial-thickness burns of 10% TBSA or more
• deep partial or full-thickness burns of the face, hands, feet, genitalia, perineum, or over a major joint
• any suspected inhalation injury, any chemical burn, high-voltage or lightning injury
• burns with other trauma, burns in people with serious medical conditions, and pain that cannot be controlled

A partial-thickness burn under 10% still gets a consultation recommended. And for children, 14 or under or under 30 kg, the guidelines say every burn may benefit from referral.

You will still see burns sorted into minor (under 10% TBSA), moderate (10 to 20%) and major (over 20%) in older material. Those bands describe the size. The referral criteria are what decide where the patient goes.

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’s Advanced Burn Life Support course has since moved to 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.
Children start at 3 mL/kg/%TBSA, aiming for about 1 mL/kg/hr of urine, and small children also need maintenance fluid with sugar in it on top. High-voltage electrical injuries start at 4 mL.
All of these 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%.