Serum Osmolality

Serum Osmolality & Tonicity Calculator

Calculated osmolality · effective tonicity · osmolal gap · Based on current electrolyte physiology guidelines

Serum Osmolality
Osm = 2×Na⁺ + K⁺ + (Urea mg/dL ÷ 2.8) + (Glucose mg/dL ÷ 18)
Effective Tonicity (Tonicité)
Ton = 2×Na⁺ + K⁺ + (Glucose mg/dL ÷ 18) — urea excluded (freely diffusible)
Osmolal Gap = Measured Osm − Calculated Osm · Normal < 10 mOsm/kg

Please enter valid values in all required fields.

Electrolytes & metabolites

Sodium Na⁺ (mmol/L) Serum sodium — primary osmole · Normal 136–145 mmol/L

Potassium K⁺ (mmol/L) Serum potassium · Normal 3.5–5.0 mmol/L

Urea (mg/dL) Blood urea nitrogen (BUN) · Normal 7–20 mg/dL · Freely diffusible (ineffective osmole)

Glucose / BSL (mg/dL) Blood sugar level · Normal fasting 70–99 mg/dL · Effective osmole

Measured osmolality (optional) Lab-measured value — required to calculate osmolal gap · mOsm/kg

Calculated Osmolality

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mOsm/kg H₂O

Effective Tonicity

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mOsm/kg H₂O

Reference ranges & clinical thresholds

Parameter	Range (mOsm/kg)	Interpretation	Clinical context
Osmolality	< 270	Hypo-osmolar	Hyponatraemia, SIADH, water excess
Osmolality	275 – 295	Normal	Physiological range
Osmolality	296 – 320	Hyper-osmolar	Dehydration, hypernatraemia, DKA
Osmolality	> 320	Critically high	Severe DKA, HHS, toxic alcohol ingestion
Osmolal gap	< 10	Normal	No unmeasured osmoles suspected
Osmolal gap	≥ 10	Elevated	Toxic alcohols (methanol, ethanol, ethylene glycol), mannitol, renal failure

Formula: Osm = 2Na⁺ + K⁺ + Urea(mg/dL)/2.8 + Glucose(mg/dL)/18 · Normal range 275–295 mOsm/kg · All calculations run locally · No data transmitted

Clinical references

1.Bhave G, Neilson EG. Body fluid dynamics: back to the future. J Am Soc Nephrol. 2011;22(12):2166–2181. doi:10.1681/ASN.2011080865
2.Hoorn EJ, Zietse R. Diagnosis and treatment of hyponatremia: compilation of the guidelines. J Am Soc Nephrol. 2017;28(5):1340–1349. doi:10.1681/ASN.2016101139
3.Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1–G47. (European joint ESE/ERA/ESCMID guideline)
4.Purssell RA, Lynd LD, Koga Y. The use of the osmole gap as a screening test for the presence of exogenous substances. Toxicol Rev. 2004;23(3):189–202.
5.Schrier RW (ed). Diseases of the Kidney and Urinary Tract. 8th ed. Lippincott Williams & Wilkins; 2007. Chapter on osmolality and tonicity.
6.Kamel KS, Halperin ML. Fluid, Electrolyte and Acid-Base Physiology. 5th ed. Elsevier; 2017. (Osmolality and tonicity framework)
7.Liamis G, Liberopoulos E, Barkas F, Elisaf M. Spurious electrolyte disorders: a diagnostic challenge for clinicians. Am J Nephrol. 2013;38(1):50–57. (Pseudohyponatraemia and calculated vs measured osmolality)
8.Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399–403. (Glucose correction for serum sodium)

Frequently asked questions

Medical disclaimer

For educational and professional reference only. This Serum Osmolality & Tonicity Calculator is intended to assist qualified clinicians — intensivists, nephrologists, endocrinologists, emergency physicians, and allied health professionals. It is not a substitute for clinical judgment, direct patient assessment, or laboratory measurement.

The formula used (Osm = 2Na⁺ + K⁺ + Urea/2.8 + Glucose/18) is one of several validated approximations. Minor variations exist in published literature — some omit K⁺, some use different divisors for urea. This calculator uses the comprehensive four-variable formula most consistent with current nephrological and intensive care references.

The calculated osmolality represents the contribution of measured osmoles only. The osmolal gap (measured minus calculated) reflects unmeasured osmoles and is clinically valuable for detecting toxic alcohol ingestion (methanol, ethanol, ethylene glycol, isopropanol), mannitol accumulation, or severe uraemia. A gap above 10 mOsm/kg warrants clinical investigation — but a normal gap does not exclude toxin ingestion, particularly methanol, where the gap may normalise as formate accumulates.

Effective tonicity (excluding urea, which freely crosses cell membranes) is the physiologically relevant measure driving transcellular water shifts and cell volume. It is used to guide fluid and sodium management in dysnatraemias.

All values should be interpreted in the context of the full clinical picture including symptoms, volume status, urinary sodium, urinary osmolality, and concurrent medications. This tool does not store, transmit, or process any patient-identifiable information. All calculations occur locally in the user's browser.

Last reviewed: January 2026 · ESE/ERA hyponatraemia guidelines 2014 (updated 2023) · Kamel & Halperin 5th ed. framework