To calculate an estimation, the total amount of substance in the body before the loss is first estimated:formula_1where:*nb = Total amount of substance before fluid loss*Osmb = Body osmolarity before loss (almost equal to plasma osmolality of 275-299 milli-osmoles per kilogram)*TBWb = Total body water before loss (approximately 60% of body weight, or using tritiated water or deuterium)2.