Serum Osmolality

What This Calculator Measures

Estimates serum osmolality from sodium, glucose, and blood urea nitrogen, supporting evaluation of electrolyte disorders, hydration status, and toxic ingestions.

How to Use

1. Enter serum sodium (mmol/L), glucose (mg/dL), and BUN (mg/dL).
2. Review the calculated osmolality (normal ≈ 275–295 mOsm/kg).
3. If a measured osmolality is available, compare it with the calculated value to derive the osmolar gap.

Interpretation Guide

Normal serum osmolality is 275–295 mOsm/kg. Values below ~275 mOsm/kg suggest dilutional hyponatremia. Values ≥296 mOsm/kg are high and ≥320 mOsm/kg are critically elevated, indicating hypernatremia, hyperglycemia, uremia, or ingestion of osmotically active substances.

Clinical Significance

Serum osmolality is a core component of evaluating sodium disorders, guides the interpretation of the osmolar gap for toxic alcohol exposure, and informs the rate of hypernatremia correction.

When to Use

• Evaluation of hyponatremia and hypernatremia
• Assessment of hydration status
• Suspected toxic alcohol ingestion (paired with the osmolar gap)
• Guiding the rate of hypernatremia correction

When Not to Use

• Without a measured osmolality when calculating the osmolar gap
• When glucose and BUN are entered in different units than mg/dL
• As a substitute for measured osmolality when the clinical question demands it

Limitations

• Requires glucose and BUN in mg/dL; results are incorrect with other units.
• Does not account for ethanol — add ethanol/4.6 for effective osmolality.
• An elevated osmolar gap requires a measured osmolality from the laboratory.

Worked Example

A 45-year-old man with poor intake and nausea has sodium 140 mmol/L, glucose 200 mg/dL, and BUN 14 mg/dL.

Calculated osmolality = 2(140) + 200/18 + 14/2.8 = 280 + 11.1 + 5 = 296.1 mOsm/kg — high, warranting a measured osmolality to assess the osmolar gap.

Evidence

Source

Emergency Medicine / Nephrology

Reference

Dorwart WV, Chalmers T. Clin Chem. 1975;21(2):190-194.

Reviewed By

MedCalcHub Clinical Team

Version

1.0

1. Dorwart WV, Chalmers T. Clin Chem. 1975;21(2):190-194.
2. Tintinalli's Emergency Medicine, 9th ed.

References

1. Dorwart WV, Chalmers T. Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations. Clin Chem. 1975;21(2):190-194. (Primary Study)
2. UpToDate. Serum osmolality. (Reference)

Frequently Asked Questions

Which Osmolality Calculator Should I Use?