Your total body surface area (TBSA) burn percentage is the sum of all affected body regions and represents the overall extent of burn injury. Burns affecting less than 10% TBSA in adults are generally considered minor and may be managed on an outpatient basis if they are superficial partial-thickness burns in non-critical areas. Burns covering 10% to 20% TBSA are considered moderate and typically require inpatient care with IV fluid resuscitation.

Burns exceeding 20% TBSA in adults (or 10% in children and elderly patients) are classified as major burns and require aggressive fluid resuscitation, typically calculated using the Parkland formula: 4 mL of lactated Ringer's solution per kilogram of body weight per percentage of TBSA burned, with half given in the first 8 hours and the remainder over the next 16 hours. These patients should be transferred to a verified burn center.

The TBSA percentage also has prognostic significance. Along with patient age and the presence of inhalation injury, TBSA is a key predictor of mortality. The Baux score (age + %TBSA) and revised Baux score provide mortality estimates. Accurate TBSA assessment is therefore critical not only for acute management but also for prognostication and family counseling.

The Rule of Nines should be used for rapid initial assessment of burn extent in adult patients presenting to the emergency department or at the scene of injury. It is most appropriate for medium to large burns where a quick TBSA estimate is needed to initiate fluid resuscitation and determine the need for burn center transfer. The American Burn Association transfer criteria include burns greater than 10% TBSA, among other factors.

This calculator is also useful when calculating fluid resuscitation requirements using the Parkland formula or other resuscitation protocols. Having an accurate TBSA estimate within the first hour of presentation is essential because delayed or inadequate fluid resuscitation worsens outcomes in major burns.

The Rule of Nines is designed for adult body proportions and is inaccurate for children, who have proportionally larger heads and smaller limbs. For pediatric patients, the Lund-Browder chart provides age-adjusted body surface area percentages and should be used instead. The Rule of Nines is also less accurate in obese patients, whose body proportions differ from the standard model.

This method estimates only the extent of the burn, not the depth. Burn depth (superficial, partial-thickness, or full-thickness) is a critical determinant of treatment and prognosis that requires separate clinical assessment. Only second-degree (partial-thickness) and third-degree (full-thickness) burns should be included in the TBSA calculation for fluid resuscitation — first-degree (superficial) burns are excluded.

TBSA estimation using the Rule of Nines has significant inter-rater variability, with studies showing that clinicians frequently overestimate burn size, particularly for scattered or irregularly shaped burns. Overestimation leads to excessive fluid resuscitation, which can cause complications such as abdominal compartment syndrome and pulmonary edema. For small or scattered burns, using the patient's palm (approximately 1% TBSA) as a reference may be more accurate.