The severity of shock can be graded 1-4 based on the physical signs. This approximates to the effective loss of blood volume. The blood volume does not have to actually be lost from the circulation as an expansion in the volume of the circulatory system (e.g. in septic shock) will render the patient proportionally hypovolaemic.

Grade 1
Up to about 15% loss of effective blood volume (~750ml in an average adult who is assumed to have a blood volume of 5 liters). This leads to a mild resting tachycardia and can be well tolerated in otherwise healthy individuals. In the elderly or those with underlying conditions such as ischaemic heart disease the additional myocardial oxygen demands may not be tolerated so well.
Grade 2
Between 15-30% loss of blood volume (750-1500ml) will provoke a moderate tachycardia and begin to narrow the pulse pressure. The time taken for the capillaries to refill after 5 seconds of pressure (capillary refill time) will be extended.
Grade 3
At 30 - 40% loss of effective blood volume (1500 - 2000 ml) the compensatory mechanisms begin to fail and hypotension, tachycardia and low urine output (<0.5ml/kg/hr in adults) are seen.