Following endorsement of serial halving by the Faculty of Pre-Hospital Care of the Royal College of Surgeons of Edinburgh this study aimed to determine whether the technique was comparable to the rule of nines in making initial assessments of body surface area burned. Ten 'casualties' were made up to represent burn victims (range 6-61%). An external panel of six consultants and one specialist registrar in plastic surgery were invited to assess the simulated casualties. They gave individual and a consensus estimate of the burned areas. One hundred and twenty-five members of local emergency services and military paramedical staff were given a brief video and slide presentation describing either the rule of nines or serial halving method of burn area assessment. These techniques were then used to assess the 10 simulated casualties, giving 1250 estimates of burn surface area. The understanding of both techniques appeared adequate in both test groups. Estimates from serial halving and rule of nines groups differed from the assessments of the external panel. No statistical difference was demonstrated between serial halving and the rule of nines as an initial assessment tool when determining disposal. Serial halving has an inherent weakness when assessing certain sizes of burn. The rule of nines requires that the assessor knows and understands the proportionate areas of the body. The mathematics of percentages and fractions appeared to confuse some assessors.