Serum Osmolality Calculator

Estimate Serum Osmolality from Sodium, Glucose and BUN

Serum Osmolality Formula

Formula

\[ \text{Osm} = 2 \times \text{Na} + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8} + \frac{\text{Ethanol}}{3.7} \]

Where:

\(\text{Osm}\) — calculated serum osmolality (mOsm/kg)
\(\text{Na}\) — serum sodium (mEq/L, identical numerically to mmol/L)
\(\text{Glucose}\) — serum glucose in mg/dL
\(\text{BUN}\) — blood urea nitrogen in mg/dL
\(\text{Ethanol}\) — serum ethanol in mg/dL (optional term; omit if not measured)

This is a bedside estimate of the concentration of osmotically active particles in blood plasma. Each solute in mg/dL is divided by its molecular weight factor to convert it into mmol/L, while sodium is doubled because it is paired with an equal number of anions.

If a measured osmolality is available, the osmolar gap is:

\[ \text{Osmolar gap} = \text{Osm}_{\text{measured}} - \text{Osm}_{\text{calculated}} \]

Serum Osmolality – Calculation Example

Example: a patient with a serum sodium of 140 mEq/L, glucose of 100 mg/dL, BUN of 15 mg/dL and no measurable ethanol.

\[ \text{Osm} = 2 \times 140 + \frac{100}{18} + \frac{15}{2.8} \]

\[ \text{Osm} = 280 + 5.6 + 5.4 = 290.9 \text{ mOsm/kg} \]

Result: the calculated serum osmolality is approximately 291 mOsm/kg, which falls within the normal reference range of 275–295 mOsm/kg. If the laboratory had also reported a measured osmolality of, say, 295 mOsm/kg, the osmolar gap would be \( 295 - 290.9 = 4.1 \) mOsm/kg — also normal.

How to Use the Calculator

Enter your Serum sodium in mEq/L (the same number applies if your report uses mmol/L).
Enter your Serum glucose and choose the unit — mg/dL or mmol/L — using the unit selector next to the field.
Enter your Blood urea nitrogen (BUN) and select its unit. If your report lists urea in mmol/L, enter that value and choose mmol/L; the calculator converts it for you.
If an ethanol level was measured, enter it in the Ethanol (optional) field with the correct unit; otherwise leave it at 0.
If the laboratory measured osmolality directly, type it into Measured osmolality (optional) to calculate the osmolar gap. Leave this field empty if you only have the calculated value.
Read the Calculated serum osmolality, the Osmolar gap (if a measured value was entered) and the interpretation text below them.

What Is Serum Osmolality?

Serum osmolality describes the total concentration of dissolved, osmotically active particles in the blood — primarily sodium and its accompanying anions, along with glucose and urea. It is expressed in milliosmoles per kilogram of water (mOsm/kg). Osmolality drives the movement of water between body compartments, so it is central to evaluating fluid balance disturbances.

Although osmolality can be measured directly by an osmometer, many clinical situations rely on a calculated estimate from routine chemistry values. The normal range is roughly 275–295 mOsm/kg. A value below 275 suggests hypo-osmolality, most often caused by hyponatremia; a value above 295 indicates hyperosmolality, commonly due to hyperglycemia, dehydration or kidney failure with elevated urea. Marked elevations above 320 mOsm/kg are medically urgent.

The osmolar gap compares the measured osmolality with the calculated estimate. A gap within about −10 to +10 mOsm/kg is considered normal. A significantly elevated gap means there are osmotically active substances in the blood that the formula does not account for — classic causes include toxic alcohol poisoning (methanol, ethylene glycol), acetone in diabetic ketoacidosis, mannitol therapy and radiocontrast agents. A large negative gap is unusual and often signals a laboratory error worth repeating.

Assumptions, Limitations and Practical Tips

The result is an estimate, not a measurement. Calculated osmolality typically runs about 5–10 mOsm/kg below a directly measured value, because the formula ignores minor solutes such as potassium, calcium and proteins.
Include ethanol when it is present. In intoxicated patients, omitting the ethanol term can falsely elevate the osmolar gap and mimic toxic alcohol ingestion. If you also suspect methanol or ethylene glycol, the remaining gap after accounting for ethanol is the clinically relevant signal.
Unit conversion matters. Glucose 1 mmol/L ≈ 18 mg/dL, urea 1 mmol/L ≈ 2.8 mg/dL BUN, and ethanol 1 mmol/L ≈ 4.6 mg/dL. Double-check which unit your lab report uses before entering values.
An isolated calculated value has limits. Interpretation should always happen alongside the clinical picture, other labs and, where available, a measured osmolality. This tool is for information only and is not a medical diagnosis.

Related tools that often complement this one: in severe hyperglycemia the sodium itself is diluted, so pair this calculator with the Corrected Sodium calculator; when treating hypernatremia, the Free Water Deficit calculator helps plan replacement; and because elevated BUN raises osmolality, the eGFR calculator is useful for assessing the underlying kidney function.

Frequently Asked Questions

What is the difference between osmolality and osmolarity?

Osmolality is the concentration of particles per kilogram of water (mOsm/kg), while osmolarity is per liter of solution (mOsm/L). Because plasma is about 93% water, the two values are numerically close and are often used interchangeably at the bedside. Laboratory osmometers report osmolality, which is why calculators use mOsm/kg. In clinical practice the distinction rarely changes interpretation.

Are there other formulas for calculating serum osmolality?

Yes. A common quick version is \( \text{Osm} = 2 \times \text{Na} + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8} \), the same formula without the ethanol term. Others include 275 + 5.3 × (glucose/18 − 4.5) for patients without azotemia, and the Smithline-Gardner variant. The ethanol-inclusive version used here fits measured values best in intoxicated patients; differences between formulas are usually small but matter when computing the osmolar gap.

Why does my calculated osmolality differ from the measured value?

Calculated values typically run about 5–10 mOsm/kg lower than osmometer measurements because the formula ignores minor solutes such as potassium, calcium, magnesium and plasma proteins. This small systematic offset is expected and is why the normal osmolar gap is defined as roughly −10 to +10 mOsm/kg rather than zero. If the discrepancy is much larger — especially with a negative gap — consider a lab error or an unmeasured substance.

What can cause a high osmolar gap besides toxic alcohol poisoning?

Besides methanol and ethylene glycol, an elevated gap can result from isopropanol (rubbing alcohol) ingestion, acetone in diabetic or alcoholic ketoacidosis, mannitol or glycerol therapy, radiocontrast agents, severe hyperlipidemia or hyperproteinemia (which also pseudodilutes sodium), and even lactic acidosis. Always interpret the gap together with the clinical picture, anion gap, and — if toxic alcohols are suspected — direct blood levels, since those require urgent treatment.

Do I need to correct sodium for high glucose before using this calculator?

No, for calculating osmolality you enter the reported sodium as measured — the glucose term already accounts for the osmotic effect of the sugar itself. Sodium correction (adding about 1.6–2.4 mEq/L per 100 mg/dL glucose above normal) is a separate concept used to assess the patient's true sodium status, for example when deciding whether hyponatremia is real. Use the Corrected Sodium calculator for that purpose.

How do I convert my lab values between mg/dL and mmol/L?

For glucose, multiply mmol/L by 18 to get mg/dL. For urea, multiply mmol/L by 2.8 to get BUN in mg/dL — this is a common trap, since European reports often list urea, not BUN, and the numbers differ by roughly a factor of three. For ethanol, 1 mmol/L ≈ 4.6 mg/dL. Sodium needs no conversion: mEq/L and mmol/L are numerically identical.

Is a serum osmolality of 300 or higher dangerous?

A value between 295 and 320 mOsm/kg indicates hyperosmolality, commonly caused by hyperglycemia, dehydration or elevated urea — usually manageable once the cause is treated. Values above 320 mOsm/kg, as in severe dehydration, uncontrolled diabetes or certain poisonings, can be medically urgent and affect brain function. These numbers should always be discussed with a healthcare provider along with the rest of your lab results and symptoms.