The serum osmolality calculator determines blood osmolarity, by using the BUN, glucose and ethanol value and the equation that delivers the mOsm/Kg answer. You can read more on this subject, discover an example calculation and the interpretation of high and low values below the form.


Where all values are blood concentrations and BUN is blood urea nitrogen level.

How does the serum osmolality calculator work?

This is a health tool designed to determine blood osmolality and uses the values of BUN (blood urea nitrogen), glucose and ethanol. This is one of the tests used by medical professionals to determine whether this concentration is low or high and to check whether the adjustment mechanisms of the body work properly.

You can discover the serum osmolarity on your own by inputting the data required in the serum osmolality calculator. The formula used is the following:

Osm= 2 * Na + BUN / 2.8 + glucose / 18 + ethanol / 4.6

This is an example calculation:

Na value: 140mEq/L

BUN value: 13mg/dL

Glucose: 76 mg/dL

Ethanol: 250 mg/dL

Result: Serum Osmolarity: 343.21mOsm/Kg

Osmolality v Osmolarity

These are both measurements describing the solute concentration in a solution. Although the first is the osmolar concentration of plasma (particles per kg) and is measured using an osmometer, the second is the estimation of osmolar concentration of plasma (particles/litre of solution – mmol/L), these are widely used interchangeably. Blood osmolality has levels between 275 and 303 mOsm/Kg.

Indications of the blood osmolality test

This is a basic test performed on a sample of blood, used to check the water/ salt balance of the body and is used to provide insight whenever the doctor considers the risk of dehydration, hyponatremia (low Na), loss of water, diabetes, stroke, head trauma, ADH secretion syndrome, poisoning with ethanol or similar.

High levels

- Dehydration - Diabetes mellitus
- Renal disease - Hyperglycaemia
- Addison’s disease - Hypercalcaemia
- Alcohol poisoning          

Low levels

- Overhydration - Lung disease, cancer
- Hypernatraemia - Central nervous system diseases


1) Erstad BL. (2003) Osmolality and osmolarity: narrowing the terminology gap. Pharmacotherapy; 23(9):1085-6.

2) Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. (2001) Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med; 38(6):653-9.

16 Mar, 2015