π§ͺ Serum Osmolality Calculator
Calculated vs measured: the osmolar gap.
For informational purposes only, not medical advice. Consult a healthcare professional.
Serum osmolality measures the concentration of dissolved particles in blood, and it can be calculated from three routine labs: double the sodium, add BUN divided by 2.8, add glucose divided by 18. When the lab also measures osmolality directly, the difference between measured and calculated is the osmolar gap, normally under 10. A wide gap means unmeasured osmoles are present: ethanol, methanol, ethylene glycol, or mannitol. This calculator runs the formula and the gap for clinical teaching and reference.
How to Use This Calculator
Enter sodium, BUN, and glucose from the metabolic panel. Add the lab-measured osmolality if you have it to get the osmolar gap.
Copy sodium in mEq/L, BUN and glucose in mg/dL from the same blood draw.
Read the calculated osmolality. Normal runs 275 to 295 mOsm/kg.
If the lab measured osmolality directly, enter it: the gap between measured and calculated hunts hidden osmoles like toxic alcohols.
How the Math Works
Calculated osmolality equals 2 times sodium plus BUN over 2.8 plus glucose over 18. With sodium 140, BUN 14, and glucose 90: 280 plus 5 plus 5 is 290 mOsm/kg. Sodium is doubled to account for its accompanying anions, mainly chloride and bicarbonate. The divisors convert BUN and glucose from mg/dL to mmol/L using their molecular weights. The osmolar gap is measured minus calculated; above 10 to 15 is abnormal and above 25 strongly suggests toxic alcohol ingestion. Ethanol contributes about 4.6 mOsm per 100 mg/dL and can be subtracted when known.
Worked Example
Sodium 140, BUN 14, and glucose 90 give a calculated osmolality of 290.0 mOsm/kg, which the calculator rates as Normal. With a measured osmolality of 292, the osmolar gap is 2.0, comfortably normal. Had the measured value been 330 with the same calculated 290, the gap of 40 would scream unmeasured osmoles and trigger toxic alcohol workup.
Reading Your Result
Calculated 275 to 295 with a small gap is normal physiology. High calculated osmolality with a normal gap points to the measured solutes: hypernatremia, uremia, or hyperglycemia, each with its own management. A wide gap with normal calculated osmolality is the toxicology clue: methanol and ethylene glycol kill, and the gap is often the first hint before levels return. Low calculated osmolality means hypo-osmolar states, usually hyponatremia, worked up by volume status and cause.
| Finding | Meaning | Think |
|---|---|---|
| Gap under 10 | Normal | No hidden osmoles |
| Gap 10-25 | Borderline | Ethanol, mannitol, CKD |
| Gap over 25 | High | Toxic alcohols likely |
Limitations
The formula estimates effective circulating osmoles imperfectly; urea crosses membranes freely so it contributes less to tonicity than the number suggests. Lab variation in any of the three inputs shifts the result. The gap is a screening clue, not a toxin identification: specific levels for methanol, ethylene glycol, and ethanol confirm. Treat first on clinical suspicion in toxicology; the gap supports, never delays.
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For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is serum osmolality?
The concentration of solute particles per kilogram of serum water, in mOsm/kg. It governs water movement across cell membranes and reflects the balance of sodium, urea, glucose, and any unmeasured osmoles. Normal is 275 to 295.
How do you calculate osmolality?
Two times sodium plus BUN divided by 2.8 plus glucose divided by 18. Sodium is doubled for its anions; the divisors convert mg/dL to mmol/L. It is one of the most-used bedside formulas in medicine.
What is the osmolar gap?
Measured osmolality minus calculated osmolality. Normally under 10. A wide gap means particles the formula does not count: ethanol, methanol, ethylene glycol, mannitol, or severe hyperlipidemia interfering with measurement.
Why does the osmolar gap matter in overdose?
Because toxic alcohols raise the gap hours before specific levels come back, and early treatment saves kidneys, vision, and lives. A gap over 25 with unexplained metabolic acidosis is treated as toxic alcohol ingestion until proven otherwise.
What is the difference between osmolality and osmolarity?
Osmolality is per kilogram of water; osmolarity per liter of solution. Clinically they are used almost interchangeably since the difference is tiny in dilute body fluids. Labs report osmolality.
Does alcohol affect the osmolar gap?
Yes. Ethanol adds about 4.6 mOsm/kg per 100 mg/dL of blood alcohol. In a drinking patient, subtract the ethanol contribution before interpreting the gap, or a benign ethanol gap will mimic a toxic one.
What causes low serum osmolality?
Usually hyponatremia: excess water relative to solute. Causes divide by volume status into hypovolemic, euvolemic (SIADH), and hypervolemic (heart failure, cirrhosis). The osmolality confirms the hypo-osmolar state; volume status finds the cause.
What causes high serum osmolality?
Water loss exceeding solute loss (dehydration, diabetes insipidus), high sodium intake, hyperglycemia, uremia, or unmeasured osmoles. Symptoms track the speed of change more than the absolute number.
How fast can osmolality be corrected?
Carefully. Rapid correction of chronic hyponatremia risks osmotic demyelination; guidelines limit rises to about 8 to 10 mEq/L per day. Hypernatremia correction has its own speed limits. The number guides but the brain sets the pace.
Is calculated osmolality the same as tonicity?
No. Tonicity counts only effective osmoles that do not cross cell membranes, mainly sodium salts and glucose. Urea crosses freely, so it raises osmolality without raising tonicity. That distinction drives fluid-choice decisions.