Metabolism
Serum osmolality calculator (calculated osmolality and osmolal gap)
Calculate serum osmolality from sodium, glucose and BUN (or urea), in US or SI units. Add a measured osmolality from your lab report to get the osmolal gap. The usual reference range for serum osmolality is 275 to 295 mOsm/kg.
Enter your values above to see your result and what it means.
How to calculate serum osmolality
The common formula (Smithline and Gardner) doubles sodium, because each sodium ion travels with a negatively charged partner, and adds glucose and urea converted to mmol/L. In US units: 2 × Na + glucose / 18 + BUN / 2.8. In SI units: 2 × Na + glucose + urea. With sodium 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL the result is 280 + 5 + 5 = 290 mOsm/kg.
| Your report shows | Formula | Example |
|---|---|---|
| BUN and glucose in mg/dL | 2 × Na + glucose / 18 + BUN / 2.8 | 2 × 140 + 90 / 18 + 14 / 2.8 = 290 |
| Urea and glucose in mg/dL | 2 × Na + glucose / 18 + urea / 6 | 2 × 140 + 90 / 18 + 30 / 6 = 290 |
| Urea and glucose in mmol/L | 2 × Na + glucose + urea | 2 × 140 + 5 + 5 = 290 |
Normal serum osmolality
Normal serum osmolality is 275 to 295 mOsm/kg, and laboratories vary slightly. Because sodium is doubled in the formula, it dominates the result: a change of 5 mmol/L in sodium moves the calculated value by 10, while glucose and urea matter only when they are well outside their own ranges.
| Osmolality | Range | Common reasons |
|---|---|---|
| Below 275 mOsm/kg | Low | Low sodium, excess water intake, SIADH, underactive thyroid |
| 275 to 295 mOsm/kg | Normal | Usual balance of sodium, glucose and urea |
| Above 295 mOsm/kg | High | Dehydration, high glucose, high sodium, urea build-up, diabetes insipidus |
The osmolal gap
The osmolal gap is the measured osmolality minus the calculated one. A gap above 10 is the cut-off most often used to suggest that something the formula does not count is present in the blood. In a study of patients tested for toxic alcohols, a gap above 10 caught most of those who needed treatment and also flagged many people who had none, often because of ordinary alcohol. So the gap is a pointer for doctors, read together with the anion gap and the clinical picture.
BUN versus urea
BUN and urea describe the same substance. BUN counts only its nitrogen, so in mg/dL the urea figure is a little over twice the BUN figure. In mmol/L they are the same number. The calculator converts whichever you enter: BUN in mg/dL is divided by 2.8 and urea in mg/dL by 6 to get mmol/L.
Frequently asked questions
What is the formula for serum osmolality?
Calculated serum osmolality = 2 × sodium + glucose / 18 + BUN / 2.8, with glucose and BUN in mg/dL. In SI units it is 2 × sodium + glucose + urea, all in mmol/L.
What is a normal serum osmolality?
275 to 295 mOsm/kg. Ranges vary slightly between laboratories, so compare with the range printed on your report.
What is the difference between osmolality and osmolarity?
Osmolality counts dissolved particles per kilogram of water and osmolarity per litre of solution. Laboratories measure osmolality. The formula technically gives osmolarity, and in blood the two are close enough that both are reported in practice.
What does a high osmolal gap mean?
A gap above 10 suggests a substance in the blood that the formula does not count. Ordinary alcohol (ethanol) in the blood is one reason. Toxic alcohols such as methanol and ethylene glycol also raise it, which is why emergency doctors calculate it when poisoning is possible.
Why is sodium multiplied by 2?
Each sodium ion is balanced by a negatively charged ion, mostly chloride and bicarbonate, and those particles count too. Doubling sodium is a shortcut for both.
Sources
- MedlinePlus Medical Encyclopedia. Osmolality blood test.
- Najem O, Shah MM, Zubair M, De Jesus O. Serum Osmolality. StatPearls. Treasure Island (FL): StatPearls Publishing; 2024.
- Lynd LD, Richardson KJ, Purssell RA, et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med. 2008;8:5.