Serum Osmolality Calculator

Uses BUN, sodium, glucose and ethanol values to determine the blood osmolality in mOsm/Kg.

Read more about the formula used and the interpretation of the serum osmolality value, in the text below the form.

The serum osmolality calculator determines the blood osmolality value based on Na, blood urea nitrogen, glucose and ethanol.

Serum osmolality is one of the determinations clinicians used to check whether blood concentration is adequate and whether adjustment mechanisms of the body are functioning properly or not.

The formula used is:

Serum Osm = 2 x Na in mEq/L + BUN in mg/dL / 2.8 + Glucose in mg/dL / 18 + Ethanol in mg/dL / 4.6

Normal blood osmolality has levels between 275 and 303 mOsm/Kg.

*Where all values are blood concentrations and BUN is blood urea nitrogen level.
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Variables and formula

Blood osmolality can provide information on the concentration of blood and on adjustment mechanisms, based on the following:

■ Na – serum sodium in mEq/L. The normal values are around 135 to 145 mEq/L.

■ BUN – blood urea nitrogen, a waste product of protein metabolism with normal blood values of around 6 to 20 mg per 100 mL.

■ Glucose – serum glucose or blood sugar normal values are between 72 and 108 mg/dL which equals to 4.0 to 6.0 mmol/L.

■ Ethanol – is the quantitative measurement of ethanol (ethyl alcohol) in the blood. Normal findings are 0 to 50 mg/dL.

The formula used is the following:

Serum Osm = 2 x Na + BUN / 2.8 + Glucose / 18 + Ethanol / 4.6


Blood osmolality guidelines

Although blood osmolality and blood osmolarity are often used interchangeably because they refer to the solute concentration in a solution, the difference between the two is in the units they are expressed in:

■ Osmolality is measured by an osmometer and is defined by the osmoles of solute per kilogram of solvent (Osm/Kg);

■ Osmolarity is the estimation of osmolar concentration of plasma per liter of solution (Osm/L).

The blood osmolality test is performed when there is suspicion of dehydration, hyponatremia, diabetes, stroke, ADH secretion syndrome or ethanol poisoning.

Normal serum osmolality levels are between 275 and 303 mOsm/Kg.

The table below introduces indications for both abnormally low and abnormally high serum osmolality:

High levels Low levels
Dehydration Overhydration
Addison’s disease Hyponatremia
Renal disease Lung disease, cancer
Hyperglycaemia or
Diabetes mellitus
Syndrome of inappropriate antidiuretic hormone secretion
Hyperglycaemia Central nervous system diseases
Hypercalcaemia Congestive heart failure
Alcohol poisoning Nephrotic syndrome


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

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

Specialty: Hematology

Objective: Determination

Type: Calculator

No. Of Variables: 4

Article By: Denise Nedea

Published On: May 29, 2017

Last Checked: May 29, 2017

Next Review: May 29, 2023