Serum Osmolality Calculator

Background.

A serum osmolality calculator estimates the total concentration of dissolved particles in blood plasma from three routine chemistry values: sodium, glucose, and blood urea nitrogen (BUN). The common bedside formula is 2 times sodium, plus glucose divided by 18, plus BUN divided by 2.8, all expressed in mOsm/kg. For a patient with sodium 138 mEq/L, glucose 108 mg/dL, and BUN 20 mg/dL, the calculated osmolality is 289.1 mOsm/kg. If a laboratory directly measures osmolality on the same sample and reports 320 mOsm/kg, the difference between that measured value and the calculated value — the osmolal gap — is 30.9 mOsm/kg, a gap large enough to raise concern for unmeasured osmotically active substances.

This calculator serves two distinct clinical use cases. The first is monitoring severe hyperglycemia, where calculated or effective osmolality is part of the diagnostic picture for diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS). The second is toxicology screening: because ethanol, methanol, ethylene glycol, isopropanol, and other small osmotically active molecules are not part of the standard calculated-osmolality formula, an elevated osmolal gap between a directly measured osmolality and the calculated value can be an early clue to their presence, well before a specific alcohol level returns from the lab.

The formula has a documented history. Dorwart and Chalmers derived the original regression-based estimate from 715 paired patient samples in 1975, using the equation 1.86 times sodium plus glucose/18 plus BUN/2.8 plus 9. Smithline and Gardner simplified that equation the following year, replacing the 1.86 coefficient with 2 and dropping the additive constant, producing the version most bedside calculators use today. More recent diagnostic-accuracy work has proposed refined coefficients for specific screening purposes, and this calculator states which version it uses rather than presenting a single number as universally exact.

The calculator also separates "effective" osmolality from the full calculated value. Effective osmolality — 2 times sodium plus glucose/18, without the BUN term — is the number used in hyperglycemic-crises consensus guidance, because urea crosses cell membranes freely and does not draw water between the intracellular and extracellular compartments the way sodium and glucose do. BUN contributes to total measured osmolality but not to the osmotic gradient that actually moves water and drives symptoms, so clinicians distinguish the two numbers rather than treating them interchangeably.

This is an educational estimate, not a diagnosis. Osmolality and osmolal gap results must be interpreted by a clinician alongside the full clinical picture — mental status, volume status, other labs, and, where relevant, directly measured toxic-alcohol levels. Do not change treatment, IV fluid choice, or monitoring frequency based on this calculator alone.

What is serum osmolality calculator?

Serum osmolality is a measure of how many dissolved particles are packed into each kilogram of blood plasma, expressed in mOsm/kg. A normal range is roughly 275 to 295 mOsm/kg. Because directly measuring osmolality requires a freezing-point-depression osmometer that is not run on every routine panel, clinicians commonly estimate it from three values that are: serum sodium, serum glucose, and blood urea nitrogen (BUN).

Two related but distinct numbers matter here. Calculated osmolality includes all three chemistry values and approximates what a direct osmometer would report. Effective osmolality (sometimes called tonicity) excludes BUN because urea moves freely across cell membranes and does not create the osmotic gradient that shifts water between body compartments — it is the number more directly tied to symptoms like altered mental status in hyperosmolar states. The osmolal gap is the difference between a directly measured osmolality and the calculated value; a gap larger than roughly 10 mOsm/kg can indicate the presence of unmeasured osmotically active substances, classically toxic alcohols, though other conditions can also raise it.

This calculator computes arithmetic from chemistry values you provide. It cannot diagnose diabetic ketoacidosis, hyperosmolar hyperglycemic state, or toxic-alcohol ingestion, and a normal calculated osmolality does not rule any of these out. Use it as one input alongside full clinical assessment and laboratory interpretation by a qualified clinician.

How to use this calculator.

1. 01Enter serum sodium in mEq/L.
2. 02Enter serum glucose and select mg/dL or mmol/L to match your lab report.
3. 03Enter blood urea nitrogen (BUN) and select mg/dL or mmol/L.
4. 04If a lab-measured osmolality is available, leave the comparison toggle on and enter it to see the osmolal gap; otherwise turn the toggle off.
5. 05Review calculated osmolality, effective osmolality, and (if applicable) the osmolal gap.
6. 06Interpret all values with the full clinical picture and a clinician's judgment — this tool does not diagnose DKA, HHS, or toxic-alcohol ingestion.

The formula.

The calculator first converts glucose and BUN to mg/dL if they were entered in mmol/L: glucose in mmol/L is multiplied by 18, and BUN in mmol/L is multiplied by 2.8 — the same factor used later to convert BUN back to its molar contribution, since 2.8 is the conventional mg/dL-per-mmol/L conversion factor for urea nitrogen.

Effective osmolality is computed first, as 2 times sodium plus glucose(mg/dL)/18. In the worked example, sodium is 138 mEq/L and glucose is 108 mg/dL, so effective osmolality is 2×138 + 108/18 = 276 + 6 = 282 mOsm/kg. Calculated osmolality adds the BUN term: BUN(mg/dL)/2.8. With BUN of 20 mg/dL, that term is 20/2.8 = 7.142857142857143, so calculated osmolality is 282 + 7.142857142857143 = 289.142857142857143 mOsm/kg, displayed as 289.1.

If a lab-measured osmolality is available and the comparison toggle is on, the osmolal gap is measured minus calculated. With a measured value of 320 mOsm/kg, the gap is 320 − 289.142857142857143 = 30.857142857142857, displayed as 30.9 mOsm/kg. A gap in this range is well above the roughly 10 mOsm/kg cutoff commonly used to flag unmeasured osmotically active substances, and would typically prompt evaluation for toxic-alcohol ingestion alongside the rest of the clinical picture. When the comparison toggle is off, the calculator returns a gap of 0 rather than leaving the field blank or undefined, and that 0 should be read as "not computed," not as a clinical finding of zero gap.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Dorwart WV, Chalmers L (1975). Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations. Clinical Chemistry 21(2):190-194.
2. [2]Smithline N, Gardner KD Jr (1976). Gaps — Anionic and Osmolal. JAMA 236(14):1594-1597.
3. [3]Lepeytre F, Ghannoum M, Ammann H, Madore F, Troyanov S, Goupil R, Bouchard J (2017). Formulas for Calculated Osmolarity and Osmolal Gap: A Study of Diagnostic Accuracy. American Journal of Kidney Diseases 70(3):347-356.
4. [4]Hoffman RS, Smilkstein MJ, Howland MA, Goldfrank LR (1993). Osmol gaps revisited: normal values and limitations. Journal of Toxicology: Clinical Toxicology 31(1):81-93.
5. [5]StatPearls / NCBI Bookshelf, National Institutes of Health. Serum Osmolality.
6. [6]Umpierrez GE, et al. (2024). Hyperglycemic Crises in Adults With Diabetes: A Consensus Report. Diabetes Care 47(8):1257-1275.
7. [7]MedlinePlus, National Library of Medicine, National Institutes of Health. Sodium blood test.