Serum Osmolality Calculator
Enter your patient's serum sodium, BUN, and glucose to calculate estimated serum osmolality using the standard Dorwart-Chalmers formula. Optionally add a serum ethanol level and the measured osmolality from the lab to compute the osmolar gap and flag the presence of unmeasured osmoles. Results update instantly.
Formula
Worked example
For a patient with Na 140 mEq/L, BUN 14 mg/dL, glucose 90 mg/dL, and no ethanol: osmolality = 2(140) + 14/2.8 + 90/18 = 280 + 5 + 5 = 290 mOsm/kg. This falls within the normal range of 275-295 mOsm/kg.
What is serum osmolality?
Serum osmolality is the concentration of dissolved particles (osmoles) per kilogram of water in the blood. It reflects the balance between solutes such as sodium, potassium, glucose, and urea and the body's total water. Osmolality is a key driver of antidiuretic hormone (ADH) secretion from the posterior pituitary: when osmolality rises above about 285 mOsm/kg, ADH is released, causing the kidneys to retain water and bring osmolality back down. When it falls, ADH is suppressed and water is excreted. The normal range for adults is 275 to 295 mOsm/kg. The calculated value this tool produces is an estimate called serum osmolarity, while the lab value measured by freezing-point depression is osmolality; the terms are often used interchangeably in clinical practice because the difference is small in healthy patients.
The formula and what each term means
The standard Dorwart-Chalmers formula for estimated serum osmolality is: 2*Na + BUN/2.8 + glucose/18. Each component is converted from its conventional unit to milliosmoles per kilogram. Sodium is multiplied by 2 to account for its paired anions (mainly chloride and bicarbonate), which together make sodium and its anions the dominant extracellular osmoles. BUN is divided by 2.8 (the molecular weight of urea nitrogen divided by 10) to convert mg/dL to mmol/L. Glucose is divided by 18 (its molecular weight divided by 10) for the same reason. A fourth term, ethanol/4.6, can be added when serum alcohol is measured; dividing by 4.6 converts mg/dL ethanol to mmol/L. In SI units, sodium (mmol/L), BUN (mmol/L for urea), and glucose (mmol/L) are already in mOsm/kg units and do not require division, simplifying the formula to: 2*Na + BUN + glucose.
Osmolar gap and why it matters
The osmolar gap is the difference between the measured osmolality from the laboratory and the calculated osmolality from this formula. A gap between -14 and +10 mOsm/kg is considered normal. A gap above +10 indicates the presence of additional osmotically active substances that the formula does not account for. The most clinically significant causes of an elevated osmolar gap are poisoning with toxic alcohols: methanol (wood alcohol), ethylene glycol (antifreeze), and isopropanol (rubbing alcohol). Other causes include mannitol infusion, propylene glycol toxicity, and severe ketoacidosis. Because methanol and ethylene glycol are metabolized to highly toxic acids, an elevated osmolar gap combined with a high anion gap metabolic acidosis is a medical emergency. Including ethanol in the formula corrects for its contribution to the gap, making the residual elevation more specific for these toxins.
Clinical uses and limitations
Clinicians use serum osmolality to evaluate electrolyte disorders, fluid status, and suspected toxic ingestions. It helps distinguish hyponatremia types: iso-osmolar pseudohyponatremia (from severe hyperlipidemia or hyperproteinemia), hypo-osmolar true hyponatremia, and hyperosmolar hyponatremia (from hyperglycemia or mannitol). In critically ill patients, trending osmolality guides fluid resuscitation. The calculated value has limitations: it assumes the dominant solutes are sodium, BUN, and glucose. It underestimates osmolality when other solutes are present (toxic alcohols, radiocontrast, sucrose). It should always be compared with the measured lab value when toxic ingestion is suspected. It is not validated in neonates or patients on renal replacement therapy without careful adjustment.
Serum osmolality interpretation
| Osmolality (mOsm/kg) | Interpretation | Common causes |
|---|---|---|
| Below 275 | Hypo-osmolality | Hyponatremia, SIADH, hypothyroidism |
| 275-295 | Normal | Euvolemic, healthy state |
| 296-320 | Mildly elevated | Dehydration, hypernatremia, hyperglycemia |
| 321-360 | Severely elevated | Diabetes insipidus, severe dehydration, hyperglycemic crisis |
| Above 360 | Critical | Impaired consciousness likely; urgent care needed |
Reference ranges for adults. Values measured in mOsm/kg H2O (equivalent to mOsm/L for dilute biological fluids).
Frequently asked questions
What is the normal serum osmolality range?
The normal range for adults is 275 to 295 mOsm/kg H2O. Values below 275 suggest hypo-osmolality, most commonly from hyponatremia. Values above 295 indicate hyperosmolality, most often from dehydration, hypernatremia, or hyperglycemia. Values above 320 mOsm/kg are considered severely elevated and can cause neurological symptoms.
What is the osmolar gap and when should I calculate it?
The osmolar gap is the measured serum osmolality (from the lab) minus the calculated osmolality (from this formula). The normal gap is -14 to +10 mOsm/kg. Calculate it whenever you suspect toxic alcohol ingestion (methanol, ethylene glycol, isopropanol), in unexplained high anion gap metabolic acidosis, or when the clinical picture does not fit the calculated value. A gap above +10 mOsm/kg suggests the presence of unmeasured osmoles.
Why is sodium multiplied by 2 in the formula?
Sodium is the dominant extracellular cation. To maintain electrical neutrality, each sodium ion is balanced by an anion (mainly chloride and bicarbonate). Multiplying sodium by 2 accounts for both sodium and its accompanying anions, giving the total contribution of the sodium-chloride-bicarbonate system to osmolality. This is an approximation; actual measured osmolality is slightly higher because potassium and other minor cations also contribute.
What is the difference between osmolality and osmolarity?
Osmolality is the number of osmoles per kilogram of solvent (mOsm/kg H2O), measured directly by the lab using freezing-point depression. Osmolarity is osmoles per liter of solution (mOsm/L), which is what the formula calculates. In dilute biological fluids like blood, the two values are nearly identical (within 1-2%) and the terms are often used interchangeably clinically. Strictly speaking, the formula gives osmolarity, not osmolality, which is why the result is compared against the measured osmolality using the osmolar gap.
Can I use this calculator for pediatric patients?
The formula uses the same electrolyte physiology in children as in adults, so the calculation is valid. However, the normal range is slightly different for neonates and young infants (typical reference ranges for neonates are 275-290 mOsm/kg). Consult pediatric-specific reference ranges and a pediatric specialist for interpretation in patients under 1 year of age.
Sources
- Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations. Clinical Chemistry, 1975.
- Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clinical Journal of the American Society of Nephrology, 2008.