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Calculated (estimated) plasma osmolality, using the standard published formula (mg/dL units): Posm = 2 x Sodium + (Glucose / 18) + (BUN / 2.8). Commonly cited normal reference range is roughly 275-295 mOsm/kg. Pure arithmetic on given lab values. For general reference and educational purposes only — not a medical diagnosis. Consult a doctor for actual fluid/electrolyte assessment; a measured (not calculated) osmolality is more accurate when available, and the difference between the two (the "osmolar gap") can itself be clinically meaningful.
How it works
Enter sodium, glucose, and BUN, all in standard mg/dL (sodium in mEq/L). The tool applies the standard calculated plasma osmolality formula.
- Enter sodium, Na (mEq/L).
- Enter glucose (mg/dL).
- Enter blood Urea Nitrogen, BUN (mg/dL).
- Click Calculate to see your results.
Examples
Na 140, Glucose 90, BUN 14
Posm = 2(140) + 90/18 + 14/2.8 = 280 + 5 + 5 = 290 mOsm/kg.
Who should use it
- Reviewing fluid and electrolyte labs alongside a doctor.
- Nephrology and emergency medicine education about osmolality and the osmolar gap.
Industry applications
- Emergency medicine and critical care reference
- Nephrology education
Advantages
- Simple, standard, widely taught formula.
- Uses three routinely available lab values.
Limitations
- An estimate — a directly measured osmolality is more accurate when available.
- Does not account for unmeasured osmotically active substances (e.g., certain toxins or alcohols).
Common mistakes to avoid
- Using sodium in the wrong units (this formula expects mEq/L, the standard reporting unit).
- Comparing the calculated value directly to a measured osmolality without considering the normal expected small gap between the two.
Best practices
- When a measured osmolality is available, compare it to this calculated estimate to help assess for an osmolar gap, with your doctor.
Tips
- A significantly elevated osmolar gap (measured minus calculated) is a well-known clue prompting evaluation for toxic alcohol ingestion in the right clinical context.