Plasma osmolality
Plasma osmolality is the concentration of dissolved particles in plasma, mainly driven by sodium, glucose, and urea-related solutes. A plasma osmolality calculator estimates calculated osmolality, shows each solute contribution, and helps explain how osmotic concentration affects water movement between body compartments.
Water tends to move toward the compartment with the higher effective osmotic concentration. Sodium and glucose are especially important for tonicity, while urea contributes to measured osmolality but is often less effective for sustained water shifts across many cell membranes.
Core formulas
In conventional units, glucose and blood urea nitrogen are entered in mg/dL. The common calculated plasma osmolality estimate is:
\[
\begin{aligned}
\text{Calculated osmolality}
&= 2\cdot[\mathrm{Na^+}]
+ \frac{\text{glucose}}{18}
+ \frac{\text{BUN}}{2.8}
\end{aligned}
\]
In SI units, glucose and urea are entered in mmol/L, so the calculation is written more directly:
\[
\begin{aligned}
\text{Calculated osmolality}
&= 2\cdot[\mathrm{Na^+}]
+ [\text{glucose}]
+ [\text{urea}]
\end{aligned}
\]
If measured osmolality is available, the osmolar gap compares the measured value with the calculated estimate:
\[
\begin{aligned}
\text{Osmolar gap}
&= \text{measured osmolality}
- \text{calculated osmolality}
\end{aligned}
\]
Sodium is entered in mmol/L. Glucose is entered in either mg/dL or mmol/L depending on the selected unit system. BUN is used in conventional units, while urea is used in SI units. Osmolality is commonly reported in mOsm/kg.
How to interpret the result
A typical teaching range for plasma osmolality is about 275–295 mOsm/kg. A lower value suggests relatively dilute plasma, while a higher value suggests more concentrated plasma. If effective osmolality is high, water tends to leave cells; if effective osmolality is low, water tends to enter cells.
The contribution breakdown shows why the value changes. Hyperglycemia raises osmolality through the glucose term, while a uremia-like pattern can raise measured osmolality through the urea or BUN term. A raised osmolar gap suggests that measured osmolality is higher than expected from sodium, glucose, and urea/BUN alone.
- Do not mix mg/dL glucose with the SI formula.
- Use BUN for conventional units and urea for SI units.
- Do not treat osmolality and tonicity as identical in every situation.
- Measured osmolality is required before calculating the osmolar gap.
Micro example: If sodium is 140 mmol/L, glucose is 90 mg/dL, and BUN is 14 mg/dL, sodium contributes 280 mOsm/kg, glucose contributes 5 mOsm/kg, and BUN contributes 5 mOsm/kg. The calculated osmolality is about 290 mOsm/kg.
This tool is useful for learning fluid balance, osmotic concentration, solute contribution, and osmolar gap interpretation. It is not a substitute for clinical judgment; advanced interpretation should connect plasma osmolality with sodium correction, anion gap analysis, and full acid-base evaluation.
