Plasma Osmolality Calculator

Calculate plasma osmolality from serum sodium, glucose, and BUN using the standard Smithline-Gardner equation, plus effective osmolality and the osmolar gap when a lab-measured value is available.

Quick Facts

Formula
2 x Na + Glucose/18 + BUN/2.8
Smithline-Gardner equation; sodium in mEq/L, glucose and BUN in mg/dL, result in mOsm/kg.
Normal range
275-295 mOsm/kg
Population reference range; some labs use slightly different cutoffs.
Osmolar gap
Normally under about 10 mOsm/kg
A larger gap points to unmeasured osmotically active substances.

Your Results

Calculated
Calculated osmolality
-
2 x Na + Glucose/18 + BUN/2.8
Effective osmolality (tonicity)
-
Excludes BUN (freely permeant)
Osmolar gap
-
Measured minus calculated
Interpretation
-
Relative to 275-295 normal range

Ready

Enter sodium, glucose, and BUN, then calculate plasma osmolality.

About the plasma osmolality calculator

Plasma osmolality measures the concentration of dissolved particles (solutes) in blood plasma, mainly sodium and its accompanying anions, glucose, and urea. Rather than requiring a direct laboratory osmometer reading, it can be estimated closely from three routine blood values using the Smithline-Gardner equation: plasma osmolality = 2 x sodium + glucose / 18 + BUN / 2.8, with sodium in mEq/L and glucose and BUN in mg/dL. The /18 and /2.8 factors convert glucose and BUN from mg/dL into mmol/L equivalents so every term is on the same osmotic scale.

Calculated vs. effective osmolality

The full formula above gives total calculated osmolality. A related figure, effective osmolality (also called tonicity), drops the BUN term: effective osmolality = 2 x sodium + glucose / 18. Urea moves freely across cell membranes, so it does not pull water between the intracellular and extracellular space the way sodium and glucose do. Effective osmolality is the number that better reflects the risk of fluid shifting into or out of cells, which is why it is used when assessing hypertonic or hypotonic states.

The osmolar gap

When a laboratory also measures osmolality directly (by freezing-point depression), subtracting the calculated value from that measured value gives the osmolar gap. A gap under about 10 mOsm/kg is typical. A larger gap means some osmotically active substance is present in the blood that the formula does not account for - common causes include ethanol, methanol, ethylene glycol, isopropyl alcohol, and mannitol. This calculator computes the gap only when a measured osmolality greater than zero is entered.

When to consult a professional

This tool performs the standard arithmetic for education and quick reference. It does not diagnose a cause for an abnormal result and does not account for lab error, sample handling, or conditions such as severe hyperlipidemia or hyperproteinemia that can affect measured sodium. Any abnormal or unexpected result should be reviewed with a healthcare provider alongside the full clinical picture and other laboratory values.

Frequently Asked Questions

What is the formula for plasma osmolality?
The most widely used formula (the Smithline-Gardner equation) is: plasma osmolality = 2 x sodium + glucose / 18 + BUN / 2.8, where sodium is in mEq/L and glucose and BUN are in mg/dL. The result is expressed in mOsm/kg.
What is a normal plasma osmolality range?
A commonly cited normal range for plasma osmolality is about 275 to 295 mOsm/kg. Values below that range suggest relatively dilute plasma (hypotonic), and values above it suggest concentrated plasma (hypertonic); either extreme can affect fluid shifts in and out of cells.
What is the osmolar gap and why does it matter?
The osmolar gap is the difference between a lab-measured serum osmolality and the calculated value from sodium, glucose, and BUN. A gap larger than roughly 10 mOsm/kg suggests unmeasured osmotically active substances are present, such as ethanol, methanol, ethylene glycol, or mannitol, and warrants further clinical evaluation.

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