Clinical & Hospital Pharmacy
Osmolar Gap Calculator
Compares measured with calculated serum osmolality to flag unmeasured osmoles such as toxic alcohols.
Osmolar gap
-0.9mOsm/kg
Normal osmolar gap
Patient values
From the same blood sample. Glucose and BUN accept mg/dL or mmol/L.
Normal 135–145 mEq/L.
Fasting normal 70–100 mg/dL (3.9–5.6 mmol/L).
Normal 7–20 mg/dL. Use urea nitrogen, not urea.
From the lab (freezing-point osmometer). Normal 275–295 mOsm/kg.
Try an example
Working
Every term converted to mg/dL first, then to its osmolar contribution.
Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8
With ethanol: + ethanol ÷ 4.6
Osmolar gap = measured osmolality − calculated osmolality
All of glucose, BUN and ethanol are in mg/dL. Dividing glucose by 18 converts it to mmol/L; dividing BUN by 2.8 converts urea nitrogen to mmol/L; ethanol ÷ 4.6 does the same for ethanol. Sodium is doubled to count its accompanying anions (chloride, bicarbonate).
An osmolar gap > 10 suggests the presence of unmeasured osmoles — common in toxic alcohol ingestion. The normal gap varies by lab (typically −10 to +10).
Common questions
Switch the glucose and BUN units to mmol/L. The calculator converts them to mg/dL (× 18 for glucose, × 2.8 for BUN) before applying the formula, so mmol/L values effectively enter at face value. Make sure the BUN value is urea nitrogen, not urea.
Ethanol is the commonest cause of a raised osmolar gap. Ticking 'Include ethanol' adds its osmolar contribution to the calculated value, so the remaining gap reflects what ethanol does not explain — a persistent gap then points toward methanol or ethylene glycol.
No. As methanol or ethylene glycol is metabolised, the gap falls while the anion gap rises. A patient presenting late can have a normal osmolar gap and a severe acidosis.
Small negative values are within the normal −10 to +10 spread. A gap below −10 usually means a measurement or transcription error — recheck the measured osmolality and that all values are from the same sample.
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