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Osmolality Calculator
Calculates estimated serum osmolality from sodium, glucose, and blood urea nitrogen (BUN) using the standard clinical formula.
Calculator
Formula
Na⁺ is serum sodium in mEq/L (multiplied by 2 to account for paired anions). Glucose is serum glucose in mg/dL (divided by 18 to convert to mmol/L). BUN is blood urea nitrogen in mg/dL (divided by 2.8 to convert to mmol/L). The result is expressed in mOsm/kg H₂O.
Source: Bhagat CI, Garcia-Webb P, Fletcher E, Beilby JP. Calculated vs measured plasma osmolalities revisited. Clinical Chemistry. 1984;30(10):1703–1705.
How it works
Osmolality is defined as the number of osmoles of solute per kilogram of solvent (water). In plasma, the major contributors to osmolality are sodium and its associated anions (chloride and bicarbonate), glucose, and urea. Each of these has a known molecular weight that allows conversion from conventional clinical units (mg/dL) to milliosmoles per kilogram. The normal reference range for serum osmolality is approximately 275–295 mOsm/kg H₂O. Values below this range indicate hypo-osmolality (often associated with hyponatremia or overhydration), while values above 295 mOsm/kg suggest hyperosmolality, which may be caused by hypernatremia, hyperglycemia, uremia, or the ingestion of toxic alcohols.
The standard formula doubles the serum sodium concentration to account for both sodium and its paired anions (primarily chloride and bicarbonate), which together make up the dominant osmotic contribution. Glucose in mg/dL is divided by 18 (its molecular weight factor) to convert to mmol/L or mOsm/kg, and BUN is divided by 2.8 for the same reason. This formula is sometimes referred to as the Calculated Osmolality or the Estimated Plasma Osmolality formula. When the calculated osmolality is compared to a directly measured osmolality from the laboratory, the difference is the Osmol Gap — a clinically significant value that, when elevated above 10–15 mOsm/kg, suggests the presence of unmeasured osmotically active substances.
This calculation is widely applied in emergency medicine, nephrology, and critical care. In a patient with altered mental status, a high osmol gap may prompt investigation for toxic alcohol ingestion (e.g., ethanol, methanol, ethylene glycol, isopropanol). In diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS), extremely elevated glucose drives the osmolality above 320 mOsm/kg, correlating with neurological symptoms. In the ICU, tracking calculated osmolality helps guide hyperosmolar therapy with mannitol for intracranial hypertension.
Worked example
Consider a patient presenting to the emergency department with confusion. Laboratory results show:
- Serum Sodium: 148 mEq/L
- Serum Glucose: 360 mg/dL
- BUN: 28 mg/dL
Step 1 — Sodium contribution: 2 × 148 = 296 mOsm/kg
Step 2 — Glucose contribution: 360 ÷ 18 = 20 mOsm/kg
Step 3 — BUN contribution: 28 ÷ 2.8 = 10 mOsm/kg
Step 4 — Total estimated osmolality: 296 + 20 + 10 = 326 mOsm/kg
This result is significantly elevated above the normal upper limit of 295 mOsm/kg, consistent with hyperosmolality likely driven by hyperglycemia and hypernatremia. If the directly measured osmolality were, say, 340 mOsm/kg, the osmol gap would be 340 − 326 = 14 mOsm/kg, which is borderline elevated and may warrant evaluation for additional osmotically active substances.
Limitations & notes
The standard two-times-sodium formula provides an estimate, not a direct measurement. Several alternative formulas exist (e.g., those including ethanol concentration or omitting BUN as an ineffective osmole), and results may differ slightly between them. BUN is sometimes excluded from the formula because urea distributes freely across cell membranes and does not contribute to effective osmolality (tonicity) — meaning it does not cause shifts in fluid between compartments. In clinical settings where tonicity is more relevant than total osmolality (e.g., assessing cellular dehydration), the effective osmolality or tonicity formula — which excludes BUN — may be preferred: Effective Osmolality = 2 × Na⁺ + Glucose/18. Additionally, this calculator uses US conventional units (mg/dL); users working with SI units (mmol/L) must use adapted formulas. This tool should not replace direct laboratory measurement in critical clinical decisions, and results must always be interpreted alongside the full clinical picture.
Frequently asked questions
What is the normal range for serum osmolality?
The normal reference range for serum osmolality is 275–295 mOsm/kg H₂O. Values below 275 suggest hypo-osmolality, while values above 295 indicate hyperosmolality. Levels exceeding 320 mOsm/kg are associated with significant clinical symptoms including confusion, lethargy, and seizures.
What is the osmol gap and why is it clinically important?
The osmol gap is the difference between the directly measured serum osmolality (from the lab) and the calculated osmolality from this formula. A gap greater than 10–15 mOsm/kg suggests the presence of unmeasured osmotically active substances, most notably toxic alcohols such as methanol, ethylene glycol, or isopropanol. It is a key diagnostic clue in cases of suspected toxic ingestion.
Why is sodium multiplied by 2 in the osmolality formula?
Sodium is the predominant extracellular cation, and it is always accompanied by anions — primarily chloride and bicarbonate — to maintain electrical neutrality. Multiplying sodium by 2 approximates the combined osmotic contribution of sodium plus its paired anions. This simplification works well in most clinical scenarios, though it introduces a small degree of approximation.
What is the difference between osmolality and osmolarity?
Osmolality is expressed per kilogram of solvent (mOsm/kg H₂O) and is what is directly measured by a laboratory osmometer. Osmolarity is expressed per liter of solution (mOsm/L). In clinical practice, these values are very close and the terms are often used interchangeably, but serum osmolality (measured or calculated) is the standard for patient care. At normal plasma protein and lipid concentrations, the difference is negligible.
Should BUN be included when calculating effective osmolality (tonicity)?
No. Effective osmolality (also called tonicity) excludes BUN because urea freely crosses cell membranes and does not exert an osmotic force that drives water movement between compartments. The formula for effective osmolality is 2 × Na⁺ + Glucose/18. This is the preferred measure when assessing whether cells are likely to shrink or swell, such as in evaluating the clinical significance of hyponatremia or guiding fluid therapy.
Last updated: 2025-01-15 · Formula verified against primary sources.