The formula taught on Resuscitation Council UK and European Resuscitation Council courses for estimating the weight of a child has not changed.
(Age + 4) x 2
This simple formula is taught because it is easy to remember and apply in an emergency setting. It is a useful first calculation, allowing treatment to be given quickly.
Complex calculations may provide greater absolute accuracy, but they increase the risks for error. Drug calculations can be altered subsequently depending on further information such as actual weight or length and the response to initial treatment.
It is important to appreciate that plasma levels of drugs will depend on numerous other factors such as age, lean body mass, method of administration, body temperature and the pathophysiology – so absolute, pinpoint accuracy is not essential.
For information, the APLS course calculates weight in a different way with separate charts for boys and girls.