Weight loss is the best measure of the degree of dehydration. Unfortunately the child’s weight at the onset of the diarrhoea is often not known. Therefore, this method of assessing the degree of dehydration is only of limited use. With ‘some’ dehydration, less than 10% of body weight is lost while 10% or more of body weight is lost with ‘severe’ dehydration. A child may lose up to 5% of body weight (and body fluid) before the signs of dehydration can be recognized.
If a child is 10% dehydrated, 10% of the body weight will have been lost as fluid in the stool or vomitus (i.e. 100 ml/kg as 1 ml of body fluid weighs 1 g).