Objectives To gather data on the ages and weights of children aged between 1 and 16 years in order to assess the validity of the current weight estimation formula ‘Weight(kg)=2(age+4)’ and the newly derived formula ‘Weight=3(age)+7’.

Design Retrospective study using data collected from paediatric attendances at an emergency department (ED).

Setting A large paediatric ED in a major UK city.

Patients 93 827 children aged 1–16 years attending the ED between June 2003 and September 2008.

Main outcome measures Percentage weight difference between the child's actual weight and the expected weight, the latter determined by ‘Weight(kg)=2(age+4)’ and by ‘Weight(kg)=3(age)+7’, in order to compare these two formulae.

Results The weights of seriously ill children were recorded in only 20.5% of cases, necessitating a weight estimate in the remainder. The formula ‘Weight=2(age+4)’ underestimated children's weights by a mean of 33.4% (95% CI 33.2% to 33.6%) over the age range 1–16 years whereas the formula ‘Weight=3(age)+7’ provided a mean underestimate of 6.9% (95% CI 6.8% to 7.1%). The formula ‘Weight=3(age)+7’ remains applicable from 1 to 13 years inclusive.

Conclusions Weight estimation is of paramount importance in paediatric resuscitation. This study shows that the current estimation formula provides a significant underestimate of children's weights. When used to calculate drug and fluid dosages, this may lead to the under-resuscitation of a critically ill child. The formula ‘Weight=3(age)+7’ can be used over a larger age range (from 1 year to puberty) and allows a safe and more accurate estimate of the weight of children today.