The calculation of osmolality is related to plasma osmolality, urine osmolality, etc. and cannot be generalized. Osmolality is a force of water absorption by solute molecules through a semipermeable membrane, the magnitude of which depends on the number of solute particles. It usually includes plasma osmolality, urine osmolality, and so on. Plasma osmolality is calculated based on potassium concentration, sodium concentration, and glucose and urea nitrogen concentration, which is approximately equal to two times the value of potassium ion concentration, plus two times the value of sodium ion concentration, plus the concentration of urea nitrogen, plus glucose concentration, and the normal plasma osmolality is usually maintained at 280-310 mmol/L. The urinary osmolality is a function of the renal excretion of water. And urine osmolality is the total number of particles in the kidney to excrete all the solutes in the urine, such as electrolytes, urea, sugars, proteins, etc. Therefore, it is used to reflect the number of particles of solute molecules and ions per unit volume of urine. The urine osmolality/plasma osmolality ratio is generally (3 to 4.5):1 in normal people.