Serum Osmolality (SI)
Enter serum electrolytes (mmol/L) and optionally measured osmolality to estimate calculated osmolality and osmolar gap.
Including K⁺ increases accuracy slightly; omit if unavailable.
Needed to compute osmolar gap. Leave blank if unavailable.
Calculated osmolality
298.0 mOsm/kg
Osmolar gap
-298.0 mOsm/kg
How to Use This Calculator
Collect SI laboratory values
Use mmol/L for sodium, potassium, glucose, and urea. Convert mg/dL values if necessary (glucose mg/dL ÷ 18, urea mg/dL ÷ 2.8).
Calculate osmolality
The calculator applies 2 × (Na⁺ + K⁺) + glucose + urea. Potassium is optional and defaults to 0 if omitted.
Compare to measured osmolality
Enter measured serum osmolality (osmometer) to obtain the osmolar gap. Gaps >10–15 mOsm/kg indicate unmeasured osmoles.
Formula
Serum osmolality (mOsm/kg) = 2 × (Na⁺ + K⁺) + Glucose + Urea (all in mmol/L)
Osmolar gap = Measured osmolality − Calculated osmolality
Normal osmolar gap ≈ −10 to +10 mOsm/kg.
Full Description
Calculated serum osmolality estimates total solute concentration using major contributors (sodium, potassium, glucose, urea). Comparing calculated to measured osmolality yields the osmolar gap, a tool for detecting unmeasured osmoles such as toxic alcohols or organic acids. Always interpret alongside acid-base status, lactate, and clinical findings, and repeat testing if results are unexpected.
Frequently Asked Questions
Why use SI units?
Many laboratories outside the US report mmol/L. This calculator simplifies calculations without unit conversion.
Do I need potassium?
Potassium contributes minimally. If omitted, the formula defaults to Na⁺ only (2 × Na⁺ + glucose + urea).
What indicates a significant osmolar gap?
An osmolar gap >10–15 mOsm/kg suggests unmeasured osmoles (methanol, ethylene glycol, propylene glycol, isopropanol, ketoacids, lactic acidosis).
How does this differ from plasma osmolality?
Both estimate tonicity. The plasma calculator uses mg/dL inputs and includes ethanol adjustment; choose the tool that matches your lab units.