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Objectives: To gather data on the ages and weights of paediatric patients between 1 and 10 years of age, and to compare these data with the current weight estimation formula weight(kg) = 2(age+4). If a significant difference was found, the data would be used to derive a more accurate formula. Design: Retrospective study using data collected from paediatric attendances at an emergency department (ED). Setting: A large ED in a major UK city, treating both children and adults. Patients: 17 244 children aged 1–10 years, attending the ED between June and December 2005. Main outcome measures: Weight difference between the measured weight and the expected weight, the latter given by weight = 2(age+4). Results: The weights of seriously ill children were recorded in only 41.5 % of cases, necessitating a weight estimate in the remainder. The formula weight = 2(age+4) underestimated children’s weights by a mean of 18.8 % (95 % confidence interval (95 % CI) 18.42 % to 19.18%). Using linear regression and analysis of each individual age group, ten new formulas were tested. Of these formulas, weight(kg) = 3(age)+7 proved the most accurate with a mean underestimate of just 2.48 % (95 % CI 2.17 % to 2.79%). Conclusions: Weight estimation remains of paramount importance in paediatric resuscitation. This stud