What this calculator does
Serum osmolality is the concentration of dissolved particles in blood. It can be calculated from a routine panel, and comparing that calculation against a laboratory measurement reveals whether something unmeasured is present.
That difference is the osmolal gap, and it is the reason to compute this at all. The calculation accounts for sodium, glucose and urea; a gap above about 10 mOsm/kg suggests some other osmotically active substance is in the blood.
The formula
Sodium is doubled to account for its accompanying anions, glucose is divided by 18 and urea nitrogen by 2.8 to convert from mg/dL to mOsm/kg. Ethanol, if measured, is divided by 3.7 and added.
| Term | Meaning |
|---|---|
| Osmolality | Dissolved particle concentration, in mOsm per kilogram of water. |
| Osmolal gap | Measured osmolality less calculated, normally under about 10 mOsm/kg. |
| Reference range | 275 to 295 mOsm/kg in most laboratories. |
The inputs explained
| Field | What to enter |
|---|---|
| Sodium (mEq/L) | Serum sodium in mEq/L. |
| Glucose (mg/dL) | Blood glucose in mg/dL. |
| Blood urea nitrogen (BUN) (mg/dL) | Blood urea nitrogen in mg/dL. |
| Ethanol (0 if not measured) (mg/dL) | Blood ethanol in mg/dL if it was measured. Leave at zero otherwise. |
| Lab-measured osmolality (0 if unknown) (mOsm/kg) | Laboratory-measured osmolality, if available. Entering it produces the osmolal gap. |
When to use it
Calculating an osmolal gap
The gap between calculated and measured is what points toward an unmeasured substance.
Assessing hyperglycaemia
Very high glucose raises osmolality substantially, which the calculation quantifies.
Understanding a panel
Seeing how three routine values combine explains what osmolality actually represents.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does glucose raise osmolality?
The same sodium and urea at three glucose levels.
| Glucose | Calculated osmolality | Reference range |
|---|---|---|
| 90 mg/dL | 290.0 mOsm/kg | 275 - 295 mOsm/kg |
| 200 mg/dL | 296.1 mOsm/kg | 275 - 295 mOsm/kg |
| 400 mg/dL | 307.2 mOsm/kg | 275 - 295 mOsm/kg |
Questions
What does a raised osmolal gap mean?
That the blood contains osmotically active substances the calculation does not account for. There are several possible causes, some of them serious, and distinguishing between them requires urgent clinical assessment rather than a calculator.
Why is sodium doubled?
Because each sodium ion is accompanied by an anion, usually chloride or bicarbonate, to maintain electrical neutrality. Doubling accounts for both without measuring the anions separately.
Are there other formulas?
Many. More than thirty have been published, and they give slightly different results, which matters because the gap is a small difference between two larger numbers. The formula used should be stated alongside any gap.
What is the difference from osmolarity?
Osmolality is per kilogram of solvent, osmolarity per litre of solution. The two are close in blood and the terms are often used interchangeably, though laboratories measure osmolality.
For sodium adjusted for high glucose, see the corrected sodium calculator. For the unmeasured ion balance, see the anion gap calculator.