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Free serum osmolality calculator using standard teaching formulas: 2×Na + glucose/18 + BUN/2.8 (US) or 2×Na + glucose + urea (SI, mmol/L). Optionally enter measured osmolality to estimate the osmolar gap.
Last updated: April 13, 2026
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Unit system
Conventional US formula
2 × Na (mEq/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8 → estimated mOsm/kg
Estimated osmolality
290.9 mOsm/kg
Typical measured serum osmolality is often near 275–295 mOsm/kg; this is an estimate from a few solutes only.
Limitations
Does not include ethanol, ketones, mannitol, or other osmoles. Some labs report slightly different reference formulas. Not for diagnosis or treatment decisions.
Switch between conventional mg/dL chemistry and mmol/L urea/glucose so you can match your lab report format.
When measured freezing-point or vapor-pressure osmolality is available, the gap highlights unexplained osmoles—always in clinical context.
Most teaching estimates focus on sodium (with its effective anion contribution), glucose, and urea—the dominant small solutes in many patients.
Alcohol and toxic alcohol workups often add ethanol terms or institution-specific nomograms; those are outside this basic calculator.
Osmolar gap thinking is often used alongside high anion gap acidosis evaluation—see our anion gap tool for the electrolyte side.
Does not replace poison control, nephrology, or ICU protocols for osmotic therapy or dialysis decisions.
Na 140, glucose 100 mg/dL, BUN 15 mg/dL.
Estimated serum osmolality
290.9 mOsm/kg
Sodium is the dominant extracellular cation; doubling its concentration approximates the osmotic contribution of sodium plus its major paired anions in many simplified models. Glucose and urea each add roughly one milliosmole per millimole once unit conversions are applied—hence the mg/dL divisors in the US form. These relationships are taught in nephrology and emergency medicine curricula as first-pass estimates, not as laboratory reference equations.
US (example):
2 × 140 + 100/18 + 15/2.8 ≈ 280 + 5.6 + 5.4 ≈ 291 mOsm/kg
If calculated osmolality is ~290 mOsm/kg but measured serum osmolality is 330 mOsm/kg, the gap is about +40—far above a typical “roughly ±10” teaching window. That pattern prompts consideration of ethanol or other osmoles, assay timing, and urgent clinical pathways—not calculator output alone.
Share with students reviewing fluid and electrolytes.
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