Serum Osmolality Calculator

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US conventional units: sodium in mEq/L, glucose and BUN in mg/dL. SI: all three in mmol/L.
Serum sodium concentration. Normal range: 135-145 mEq/L.
mEq/L
Blood urea nitrogen. Normal range: 7-20 mg/dL (2.5-7.1 mmol/L). BUN contributes equally to osmolality on both sides of the cell membrane.
mg/dL
Serum glucose. Normal fasting range: 70-100 mg/dL (3.9-5.6 mmol/L). In hyperglycemia, each 18 mg/dL rise adds 1 mOsm/kg.
mg/dL
Leave at 0 if not measured. Legal driving limit in most US states is 80 mg/dL. Adding ethanol to the calculation reduces the apparent osmolar gap from alcohol ingestion.
mg/dL
Lab-reported serum osmolality from a freezing-point depression measurement. Enter this to calculate the osmolar gap.
mOsm/kg
Calculated OsmolalityNormal
290mOsm/kg

Estimated serum osmolality from the standard formula

Sodium contribution280mOsm/kg
BUN contribution5mOsm/kg
Glucose contribution5mOsm/kg
Ethanol contribution0mOsm/kg
290 mOsm/kg
Hypo-osmolality<275Normal275-295Mildly elevated295-320Severely elevated320+
Sodium (2xNa)280
BUN5
Glucose5
Ethanol0

Calculated serum osmolality is 290.0 mOsm/kg.

  • Calculated osmolality falls within the normal range of 275 to 295 mOsm/kg.
  • Sodium accounts for 97% of the total osmolality, glucose 2%, and BUN 2%.

Next stepSerum osmolality appears normal. Correlate with clinical picture and repeat labs as indicated.

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