How to use the calculator
The tool is intentionally simple. Enter the three lab values exactly as reported in the expected units, then calculate. Sodium usually carries the largest weight
in the result, while glucose and BUN add smaller converted contributions. That is why even modest changes in sodium can shift the estimate noticeably, whereas glucose
and BUN matter most when they are clearly elevated.
- Enter Sodium in mEq/L.
- Enter Glucose in mg/dL.
- Enter BUN in mg/dL.
- Select Calculate Osmolality to display the estimated value in mOsm/kg.
- Use Copy Result to copy the numeric result for notes or documentation.
Tip: if you also have a measured serum osmolality from the lab, compare it with the calculated value from this page to estimate an
osmolar gap (measured minus calculated). Many references consider a gap under about 10 mOsm/kg to be common, but interpretation depends on the lab method,
timing, and the rest of the clinical picture.
The calculator uses a widely taught clinical approximation for calculated serum osmolality:
It helps to read the equation as three parts. First, sodium is doubled because it is usually accompanied by anions such as chloride and bicarbonate. Second, glucose is
divided by 18 to convert the usual U.S. lab unit of mg/dL into an approximate osmotic contribution. Third, BUN is divided by 2.8 for the same kind of unit conversion.
The result is an estimate in mOsm/kg.
- Na is sodium in mEq/L.
- Glucose/18 uses glucose in mg/dL.
- BUN/2.8 uses BUN in mg/dL.
Different textbooks and institutions sometimes use slightly different constants or expanded formulas, especially when ethanol is part of the assessment.
This page follows the standard teaching version shown above. If your laboratory reports SI units, convert first rather than entering them directly.
Unit reminder: this calculator expects sodium in mEq/L and glucose/BUN in mg/dL.
If you enter mmol/L values without converting, the output will be wrong. If you are unsure, check the units printed next to the lab value or reference range.
Worked examples
The examples below are meant to make the arithmetic easy to follow. They are not treatment advice. Their value is that they show which term is driving the total and how a
change in one lab can shift the estimate.
Example 1: Typical values
Suppose a chemistry panel shows:
- Sodium (Na) = 140 mEq/L
- Glucose = 90 mg/dL
- BUN = 14 mg/dL
Step by step:
- 2 × Na = 2 × 140 = 280
- Glucose/18 = 90/18 = 5
- BUN/2.8 = 14/2.8 = 5
Estimated serum osmolality = 280 + 5 + 5 = 290 mOsm/kg. That sits within many commonly cited adult reference ranges, though every lab should still be interpreted by its own range.
Example 2: Marked hyperglycemia
Now consider a case in which glucose is high enough to matter substantially:
- Sodium (Na) = 132 mEq/L
- Glucose = 540 mg/dL
- BUN = 22 mg/dL
Step by step:
- 2 × Na = 2 × 132 = 264
- Glucose/18 = 540/18 = 30
- BUN/2.8 = 22/2.8 ≈ 7.86
Estimated serum osmolality ≈ 264 + 30 + 7.86 = 301.86 mOsm/kg. This is a useful reminder that hyperglycemia can raise osmolality even when sodium itself is not elevated.
Example 3: Low sodium with low osmolality
In many hypo-osmolar states, sodium is the dominant reason the total is low:
- Sodium (Na) = 118 mEq/L
- Glucose = 100 mg/dL
- BUN = 10 mg/dL
Step by step:
- 2 × Na = 2 × 118 = 236
- Glucose/18 = 100/18 ≈ 5.56
- BUN/2.8 = 10/2.8 ≈ 3.57
Estimated serum osmolality ≈ 236 + 5.56 + 3.57 = 245.13 mOsm/kg. In real practice, that number would be interpreted together with symptoms, volume status, urine osmolality, urine sodium, and the pace of change.
Interpretation guide (general)
A commonly quoted adult reference range is about 275 to 295 mOsm/kg, but this is only a starting point. Lab methods differ, patient context matters, and a calculated estimate is not a diagnosis by itself.
The table below is best used as orientation rather than a decision rule.
The safest way to interpret a result is to ask what is pushing it: sodium, glucose, BUN, or something not captured by the formula. That framing is especially helpful when the clinical picture does not fit the calculated number.
When the gap is unexpectedly high, clinicians may think about ethanol and other alcohols, ethylene glycol, methanol, isopropanol, propylene glycol, mannitol, sorbitol, or contrast exposure.
The gap is useful, but not perfectly specific. Timing, assay method, metabolism of the ingested substance, and individual variation can all change the picture.
A practical workflow is simple: calculate the expected osmolality here, subtract that from the measured laboratory osmolality, and then interpret the difference alongside acid-base status,
kidney function, anion gap, symptoms, and exposure history.
Calculated osmolality is convenient because it uses routine labs, but convenience is not the same as completeness. The formula intentionally simplifies a more complicated physiologic reality.
This calculator is intended for educational and informational use. If there is concern for a toxic ingestion, a serious electrolyte disorder, severe confusion, seizures, or other urgent symptoms,
direct medical evaluation is more important than any formula alone.
Sodium in plasma is paired with anions, mainly chloride and bicarbonate. Doubling sodium approximates the osmotic contribution of those sodium salts.
Not exactly. Urea increases measured osmolality, so it is included in the standard formula, but it does not behave like sodium when you think about effective tonicity and water shifts across cell membranes.
That difference is one reason the clinical interpretation can be more nuanced than the arithmetic.
The calculation itself is the same, but the interpretation may not be. Pediatric, pregnancy, dialysis, toxicology, and critical care situations often require institution-specific guidance and direct clinical judgment.
First, re-check the units and your data entry. After that, think about unmeasured osmoles, lab artifact, or timing. A measured osmolality, blood gas, anion gap, and repeat chemistry panel may be necessary in urgent cases.
