Plasma Osmolality Calculator - Calculate Blood Osmolality
Calculate plasma osmolality from sodium, glucose, and BUN to assess hydration status, electrolyte balance, and diagnose osmolality-related conditions.
Enter serum sodium (mmol/L), blood glucose (mg/dL), and blood urea nitrogen (BUN, mg/dL) to compute plasma osmolality using the standard clinical formula.
Plasma Osmolality Calculator - Calculate Blood Osmolality
Calculate plasma osmolality from sodium, glucose, and BUN to assess hydration status, electrolyte balance, and diagnose osmolality-related conditions.
About the Plasma Osmolality Calculator
Plasma osmolality is one of the most clinically important laboratory values in medicine, reflecting the total concentration of all dissolved solutes — including electrolytes, proteins, glucose, urea, and other small molecules — per kilogram of water in the blood. Expressed in milliosmoles per kilogram (mOsm/kg), it is tightly regulated by the body through a sophisticated feedback loop involving antidiuretic hormone (ADH, also called vasopressin), thirst, and the kidneys. Normal plasma osmolality ranges from 275 to 295 mOsm/kg, and even small deviations from this narrow range can have serious physiological consequences.
The standard formula used to estimate plasma osmolality is: Osmolality = 2 × [Na+] + [Glucose]/18 + [BUN]/2.8. Sodium is multiplied by 2 because it is accompanied in plasma by negatively charged anions (primarily chloride and bicarbonate) that contribute approximately equally to osmolality. Glucose is divided by 18 to convert from mg/dL to mmol/L (since glucose has a molecular weight of 180 g/mol, and 1 mmol/L = 18 mg/dL). Blood urea nitrogen (BUN) is divided by 2.8 because urea has a molecular weight of 28 g/mol (nitrogen makes up half of urea's mass), so 1 mmol/L = 2.8 mg/dL for BUN. Each of these three components contributes distinct clinical information.
Sodium is the primary determinant of plasma osmolality under normal physiological conditions. Hypernatremia (elevated sodium) almost always indicates either water deficit — as in inadequate fluid intake, diabetes insipidus, or excessive sweating — or hypotonic fluid losses. Hyponatremia (low sodium) may reflect excess free water, as in SIADH, heart failure, cirrhosis, or hypothyroidism, or true sodium deficit. Even when sodium is normal, hyperglycemia can significantly elevate osmolality. In diabetic ketoacidosis (DKA) or hyperglycemic hyperosmolar state (HHS), glucose values may exceed 500–1000 mg/dL, contributing 28–56 mOsm/kg above normal.
The osmol gap — the difference between measured and calculated osmolality — is a critical diagnostic tool. A normal osmol gap is less than 10 mOsm/kg. An elevated osmol gap suggests the presence of unmeasured osmoles, which may include toxic alcohols (ethanol, methanol, ethylene glycol, isopropanol), mannitol, or other exogenous substances. In the emergency setting, an elevated osmol gap in a patient with altered mental status and metabolic acidosis should prompt immediate consideration of toxic alcohol ingestion.
Clinically, plasma osmolality guides fluid and electrolyte management across a wide range of conditions. In neurocritical care, hyperosmolar therapy with mannitol or hypertonic saline is titrated to a target osmolality to manage cerebral edema and elevated intracranial pressure. In the intensive care unit, osmolality monitoring helps guide resuscitation in septic shock, burns, and trauma. In outpatient endocrinology, it assists in diagnosing and monitoring diabetes insipidus, SIADH, and primary polydipsia. In nephrology, it provides insights into tubular function and urinary concentrating ability when compared with urine osmolality.
This calculator uses the standard Dorwart-Chalmers formula and is validated for routine clinical use. Note that the formula estimates rather than measures osmolality; direct measurement by freezing point depression is required for diagnostic accuracy in cases where the osmol gap is clinically important. Values should always be interpreted alongside the full clinical presentation, medication history, and other laboratory parameters.
Plasma Osmolality Calculation Examples
Common clinical scenarios illustrating how sodium, glucose, and BUN contribute to plasma osmolality.
| Input Values | Osmolality | Clinical Context |
|---|---|---|
| Na+ 140 mmol/L, Glucose 100 mg/dL, BUN 15 mg/dL | 290.9 mOsm/kg | Normal osmolality in a healthy, euhydrated adult. Sodium contributes 280, glucose 5.6, BUN 5.4 mOsm/kg. |
| Na+ 155 mmol/L, Glucose 95 mg/dL, BUN 25 mg/dL | 324.2 mOsm/kg | Elevated osmolality from hypernatremia, consistent with significant dehydration. Urgent fluid replacement warranted. |
| Na+ 135 mmol/L, Glucose 400 mg/dL, BUN 18 mg/dL | 298.6 mOsm/kg | Hyperglycemia elevates osmolality despite borderline low sodium. Consistent with poorly controlled diabetes mellitus. |
| Na+ 142 mmol/L, Glucose 110 mg/dL, BUN 60 mg/dL | 311.5 mOsm/kg | Elevated osmolality driven by high BUN (uremia). Consistent with acute kidney injury or chronic kidney disease. |
How to Use the Plasma Osmolality Calculator
- Obtain the patient's serum sodium (Na+) in mmol/L from a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).
- Enter the blood glucose value in mg/dL. For fasting accuracy, use a fasting glucose measurement when possible.
- Enter the blood urea nitrogen (BUN) value in mg/dL from the same metabolic panel to ensure all values are contemporaneous.
- Click 'Calculate Osmolality' to see the estimated plasma osmolality, contribution of each component, and clinical interpretation against normal ranges.
- Compare the calculated value with a directly measured osmolality (if available) to assess the osmol gap. A gap above 10 mOsm/kg may indicate unmeasured osmoles.
Plasma Osmolality Calculator FAQ
What is the normal range for plasma osmolality?
Normal plasma osmolality is 275–295 mOsm/kg. Values below 275 mOsm/kg indicate hypoosmolality, which may reflect overhydration, SIADH, or other conditions causing dilutional hyponatremia. Values above 295 mOsm/kg indicate hyperosmolality, typically due to dehydration, hypernatremia, hyperglycemia, or uremia.
What is the osmol gap and why does it matter?
The osmol gap is the difference between the directly measured osmolality (by freezing point depression) and the calculated osmolality from this formula. A normal gap is less than 10 mOsm/kg. An elevated gap greater than 10–20 mOsm/kg suggests the presence of unmeasured osmoles, which should prompt consideration of toxic alcohol ingestion (methanol, ethylene glycol, isopropanol) or mannitol administration.
Why is sodium multiplied by 2 in the formula?
Sodium is the principal extracellular cation and is paired with anions — primarily chloride and bicarbonate — which are present in roughly equal concentration. Multiplying sodium by 2 accounts for both the sodium ions and their accompanying anions, which together constitute the majority of plasma osmolality under normal conditions.
How does hyperglycemia affect plasma osmolality?
Each increase of 18 mg/dL in blood glucose adds approximately 1 mOsm/kg to plasma osmolality. In severe hyperglycemia, as in diabetic ketoacidosis or hyperosmolar hyperglycemic state, glucose can contribute 30–50+ mOsm/kg above normal. Severely elevated glucose also causes dilutional hyponatremia, as water shifts from cells into the extracellular space.
Can this calculator be used to diagnose SIADH?
This calculator can identify hypoosmolality, which is a necessary condition for SIADH diagnosis, but SIADH requires additional criteria: hyponatremia, inappropriately concentrated urine (urine osmolality > 100 mOsm/kg), elevated urine sodium (> 20 mmol/L), and exclusion of other causes of hyponatremia such as hypothyroidism, adrenal insufficiency, and diuretic use. Always use clinical judgment alongside calculated osmolality.
What is the difference between osmolality and osmolarity?
Osmolality measures solute concentration per kilogram of solvent (mOsm/kg H₂O), while osmolarity measures per liter of solution (mOsm/L). In clinical practice, osmolality is preferred because it is independent of temperature and pressure, measured directly by freezing point depression, and is more physiologically relevant. For most clinical applications the numerical difference between the two is small and clinically insignificant.