Hypovolaemia is the leading cause of circulatory failure in children. Effective fluid resuscitation is a mainstay of patient management and is dependent on accurate detection of blood loss or volume depletion. Calculation of blood volume in children is based on age, weight and clinical physiology and the estimation of the volume of blood lost requires interpretation of the history and orthostatic vital signs, especially heart rates. Administration of fluids following these calculations will also be discussed.