Osmolality Calculator

Enter your lab values — Serum Sodium (Na), Glucose (Glu), and BUN (Blood Urea Nitrogen) — and the Osmolality Calculator computes your predicted serum osmolality in mOsm/kg. You also get the osmolal gap if you provide a measured osmolality value, helping identify potential toxins or abnormal compounds in the blood.

Disclaimer: This tool is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any health-related decisions.

mEq/L

Normal range: 135–145 mEq/L

mg/dL

Fasting normal range: 70–100 mg/dL

mg/dL

Normal range: 7–20 mg/dL

mOsm/kg

Enter the lab-measured osmolality to calculate the osmolal gap

Results

Predicted Serum Osmolality

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Osmolality Status

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Osmolal Gap

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Osmolal Gap Interpretation

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Frequently Asked Questions

What is the formula used to calculate serum osmolality?

The standard formula is: Osmolality = 2 × Na + (Glucose / 18) + (BUN / 2.8). Sodium is multiplied by 2 to account for its accompanying anions. Glucose and BUN are divided by their molecular weight conversion factors to express them in mOsm/kg units.

What is the normal range of serum osmolality?

The normal range of serum osmolality is 285–295 mOsm/kg. Values below this range indicate hypo-osmolality, while values above indicate hyperosmolality. Your measured lab value should not exceed the predicted value by more than 10 mOsm/kg.

What is the osmolal gap and why does it matter?

The osmolal gap is the difference between the measured osmolality (from the lab) and the predicted osmolality (calculated from the formula). A gap greater than 10 mOsm/kg suggests the presence of unmeasured osmotically active substances, such as ethanol, methanol, ethylene glycol, or mannitol — often toxins.

What causes an elevated osmolal gap?

Common causes include ingestion of toxic alcohols (ethanol, methanol, ethylene glycol, isopropanol), mannitol administration, severe renal failure, or high-dose lorazepam infusions (due to propylene glycol). Identifying the cause quickly is critical in emergency medicine.

Can an elevated osmolal gap indicate propylene glycol toxicity?

Yes. Propylene glycol, used as a diluent in IV lorazepam, can accumulate — especially with continuous infusions — leading to hyperosmolarity and sometimes hyperosmolar coma. An unexplained osmolal gap in ICU patients on lorazepam infusion should raise this suspicion.

What units should I use when entering values?

Enter Serum Sodium in mEq/L, Glucose in mg/dL, and BUN in mg/dL. These are the standard units used in the conventional osmolality formula. If your lab reports glucose or BUN in mmol/L, convert them first (multiply mmol/L by 18 for glucose and by 2.8 for BUN).

Is the calculated osmolality the same as the measured osmolality?

Not necessarily. The calculated (predicted) osmolality estimates what your osmolality should be based on the three main contributors: sodium, glucose, and BUN. The measured osmolality (from the lab using freezing-point depression) may be higher if other osmoles are present, which defines the osmolal gap.

What does it mean if my osmolality is above 295 mOsm/kg?

A predicted osmolality above 295 mOsm/kg suggests hyperosmolality, which can be caused by hypernatremia, hyperglycemia (such as in diabetic ketoacidosis or HHS), or elevated BUN from renal failure. Clinical symptoms can include thirst, confusion, and in severe cases, seizures or coma.