Serum Osmolality Calculator

Calculate serum osmolality from sodium, glucose, and BUN.

For educational and clinical reference. Not a substitute for medical judgment. See the medical disclaimer.
Calculated Osmolality
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References

  1. Worthley LI, Guerin M, Pain RW. For calculating osmolality, the simplest formula is the best. Anaesth Intensive Care. 1987;15(2):199-202.

How to use

  1. Enter sodium (mEq/L), glucose (mg/dL), and BUN (mg/dL).
  2. Calculated osmolality appears in mOsm/kg.
  3. Compare against measured osmolality to get the osmolar gap.

Frequently asked questions

Formula?

Calculated osmolality = 2 × Na + Glucose/18 + BUN/2.8 (mOsm/kg).

Normal range?

Normal serum osmolality is 275–295 mOsm/kg.

What is the osmolar gap?

Osmolar gap = measured osmolality − calculated osmolality. A gap > 10 mOsm/kg suggests unmeasured solutes (e.g., ethanol, methanol, ethylene glycol).

When to use this calculator?

Useful in toxic ingestions, hyponatremia workup, and ICU/critical care assessment.

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