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Osmolality Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Dorwart-Chalmers equation (1975). Last verified 14 June 2026.

Calculated serum osmolality estimates the total concentration of dissolved particles in blood plasma from routine chemistry values: sodium, glucose, and blood urea nitrogen (BUN). The most widely used clinical formula is the Dorwart-Chalmers equation: Osm = 2*Na + Glucose/18 + BUN/2.8 (in US conventional units). If a measured osmolality is also available, the calculator computes the osmol gap to help detect unmeasured osmoles (ethanol, methanol, ethylene glycol, or other toxins). This calculator is for educational and research reference. All clinical decisions require laboratory measurement and physician interpretation.

Normal: 136 to 145 mEq/L
Fasting normal: 70 to 99 mg/dL
Normal: 7 to 20 mg/dL
From lab (freezing point depression); leave blank if not available
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Osmolality formula (Dorwart-Chalmers, 1975)

Calculated Osm = 2 * Na + Glucose/18 + BUN/2.8

Osmol gap = Measured Osm - Calculated Osm

Where Na is serum sodium in mEq/L, Glucose is in mg/dL (divide by 18 to convert to mmol/L), and BUN is in mg/dL (divide by 2.8 to convert to mmol/L). The factor 2 on sodium accounts for chloride and bicarbonate as accompanying anions. Source: Dorwart B.B. and Chalmers L. (1975), Annals of Internal Medicine.

Clinical interpretation

  • Normal serum osmolality: 285 to 295 mOsm/kg
  • Hyperosmolality (greater than 295): dehydration, hypernatraemia, hyperglycaemia, azotaemia, mannitol
  • Hyposmolality (less than 280): hyponatraemia (SIADH, hypothyroidism, adrenal insufficiency)
  • Osmol gap greater than 10: suggests ethanol, methanol, ethylene glycol, isopropanol, or acetone; toxicological screening indicated

This calculator is for educational purposes. Clinical management requires physician interpretation of results in context.

Osmolality: frequently asked questions

What is osmolality?

Osmolality is the concentration of osmotically active particles (osmoles) per kilogram of solvent (mOsm/kg). Serum osmolality reflects the concentration of dissolved solutes in blood plasma and is tightly regulated between 285 and 295 mOsm/kg in healthy adults.

What is the formula for calculated serum osmolality?

The most widely cited formula (Dorwart and Chalmers, 1975) is: Osm = 2 * Na + Glucose/18 + BUN/2.8, where Na is serum sodium in mEq/L, glucose is in mg/dL, and BUN (blood urea nitrogen) is in mg/dL. This is the formula used in most US clinical laboratories.

What is the osmol gap?

The osmol gap is the difference between the measured osmolality (by freezing point depression) and the calculated osmolality. A normal osmol gap is less than 10 mOsm/kg. An elevated osmol gap (greater than 10) suggests the presence of unmeasured osmoles such as ethanol, methanol, ethylene glycol, or mannitol.

What is the normal range for serum osmolality?

The normal reference range for serum osmolality is 285 to 295 mOsm/kg H2O for adults. Values below 280 indicate hyposmolality (hyponatraemia is the most common cause); values above 295 indicate hyperosmolality (hypernatraemia, hyperglycaemia, or azotaemia).

Can this calculator be used for urine osmolality?

No. Urine osmolality must be measured directly by the laboratory (freezing point depression osmometry). Calculated osmolality formulas apply only to serum. Urine osmolality normally ranges from 50 to 1,200 mOsm/kg depending on hydration status.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.