How is the "Percentage of Burned Area" Calculated?

Estimation of the size of the burn is done using the "rule of nines" shown in Figure 1.  The corresponding area of burn is identified on the diagram, and the total percentage is calculated as the sum of the burned areas. 

The type or thickness of the burn can also be recorded on the diagram.  Many burns are mixed in nature, for example, the outer edges may be partial thickness, while areas of deep partial and full thickness burns may extend toward the center.  The appearance of the burn, along with the presence of blanching, blisters and pain helps to determine the extent of the injury. 

The percentage of burns may change as the tissue injury evolves and more extensive tissue damage becomes evident.  Infection can increase the severity of the burn over the course of the injury.

Moderate to Severe Burns Include (Dynamed):

  • third-degree (full-thickness) burns, unless very small (< 1% total body surface area) in patients aged 10-50 years and do not involve face, hands, perineum, genitals, feet, or cross a joint
  • second-degree (partial-thickness) burns that
    • cross a major joint
    • involve face, hands, perineum, genitals, or feet
    • involve > 10% of total body surface area in patients aged 10-50 years
    • involve > 5% of total body surface area in patients < 10 years old or > 50 years old
    • any burn that involves concomitant injury or severe trauma
    • any burn unlikely to heal within 3 weeks or unlikely to heal without hypertrophic scarring or functional limitation
    • fourth-degree burn


Figure 1: Rule of Nines


Determination of Burn Type

Management and Monitoring of the Burned Patient



Brenda Morgan.  (October 23, 2000)

Last Update: January 15, 2019


Buck, D., Fedorowicz, Z., and Ehrlich, A. (2018). Major burns. DynaMed updated October 23, 2018.

Burn classification - burns are acute injuries. White Paper. American Burn Association.