Serum Osmolality Calculator
Calculated plasma osmolality from sodium, glucose, and BUN
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Working out calculated plasma osmolality from sodium, glucose, and BUN is easier with Serum Osmolality Calculator, which answers as you type. Type Sodium (mEq/L), Glucose (mg/dL) and BUN (mg/dL) into Serum Osmolality; each keystroke re-runs the math. Serum Osmolality puts the headline number first. Serum Osmolality suits budgeting and planning checks. Nothing is uploaded while Serum Osmolality works, and the figures stay put. Looking for serum osmolality calculator sodium glucose bun calculated plasma osm or free online serum osmolality calculator? Serum Osmolality covers both. Keep Serum Osmolality bookmarked for next time. Serum Osmolality is free, instant and local.
What the defaults give
Sodium 140 mEq/L, glucose 90 mg/dL and BUN 14 mg/dL calculate to 290 mOsm/kg — squarely in the normal 275–295 range.
The calculation adds three contributions: sodium and its accompanying anions, glucose, and urea. The first dominates, which is why osmolality tracks sodium so closely.
Calculated osmolality and the osmolar gap
Osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8
Sodium 140, glucose 90, BUN 14: 280 + 5 + 5 = 290 mOsm/kg, inside the usual 275–295 reference range.
| Sodium | Osmolality (glucose 90, BUN 14) | Position |
|---|---|---|
| 130 | 270 mOsm/kg | below range |
| 140 | 290 mOsm/kg | within range |
| 150 | 310 mOsm/kg | above range |
The divisors convert units: glucose 180 mg/dL is 1 mmol/L because its molecular weight is 180, and urea nitrogen is divided by 2.8 rather than 28 because BUN reports nitrogen, not urea. The osmolar gap is measured osmolality minus this calculated value; a gap above about 10 suggests osmoles the formula does not count, such as mannitol or a toxic alcohol.
Related: anion gap and Winter’s formula.
The formula
Osm = 2 × Na + glucose/18 + BUN/2.8.
2 × 140 = 280. Glucose 90 ÷ 18 = 5. BUN 14 ÷ 2.8 = 5. Total = 290 mOsm/kg.
The factor of 2 on sodium accounts for the anions that accompany it — chloride and bicarbonate — which are not measured separately in this formula.
Worked example
Sodium 140 → 2 × 140 = 280
Glucose 90 mg/dL → 90 ÷ 18 = 5
BUN 14 mg/dL → 14 ÷ 2.8 = 5
Total = 290 mOsm/kg
The osmolal gap
Measured osmolality minus calculated osmolality is the osmolal gap. A gap above about 10 mOsm/kg suggests an unmeasured solute: mannitol, ethylene glycol, methanol, isopropanol or a high globulin level. In a suspected overdose the gap is often the first abnormal result, which is why the calculated figure exists as a screening tool.
What the estimate cannot do
- It is not a measurement. Freezing-point depression is the reference method; this formula estimates it.
- It ignores potassium and calcium. Their contribution is small but real, which is part of why the calculated value runs slightly low.
- It uses total BUN, not urea nitrogen in SI units. The divisors 18 and 2.8 belong to mg/dL; in mmol/L the coefficients change.
- Lipids and protein raise measured osmolality without changing the calculated figure, so a small gap is normal in samples with high triglycerides.
Where it is used
Assessing hyper- and hyponatraemia, screening for toxic alcohol ingestion, and monitoring mannitol therapy. In each case the trend and the gap matter more than the absolute figure.
Frequently Asked Questions
Why does 140 sodium give 280 of the 290 total?
Because sodium and its accompanying anions are the dominant osmoles in plasma. The factor of 2 accounts for the anions, and glucose and urea add only 5 each here.
What is a normal calculated osmolality?
Roughly 275–295 mOsm/kg. The default result of 290 sits comfortably inside that band.
What does a large osmolal gap mean?
An unmeasured solute — mannitol, ethylene glycol, methanol or isopropanol are the classic causes. A gap above about 10 mOsm/kg is the usual threshold for concern.
Why divide glucose by 18?
Because 18 is the molecular weight of glucose per unit of osmoles in mg/dL, converting the concentration into milliosmoles. BUN uses 2.8 for the same reason.
Is this the same as measured osmolality?
No. Measured osmolality comes from freezing-point depression in the lab; this formula estimates it from three routine chemistry values. The difference between them is the gap.
How much does a sodium of 150 raise the result?
Each mEq/L of sodium contributes 2 mOsm/kg, so 150 adds 300 from sodium alone. Before glucose and urea, that is already above the normal range — which is why sodium drives the figure and why osmolality changes track sodium so closely.
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