Serum sodium (Na) concentration more than 145 mmol/L is defined as hypernatremia. Hypernatremia carries an acute morbidity and mortality, especially in neonates and infants. Rapid correction of hypernatremia leads to severe cerebral damages, and seizure is the most common neurological complication of hypernatremia. Selection and calculation of an appropriate fluid and its amount is one of the tremendous challenges. It is important to choose the correct amount of suitable fluid and adjust rehydration rate. In this study, we have suggested practical equation to determine the amount of fluid that should be administered to the neonates and infants with hypernatremia. In this equation, the amount of infusate volume will be calculated from changes in serum Na and total body water. However, serum Na and infusate Na will play an important role in equation structure, too. Our suggested equation will help physicians to manage hypernatremic dehydrated patients in a suitable manner with an adjustable rate.