Serum Osmolality Calculator
Serum osmolality reflects the concentration of solutes in plasma. Enter the three major contributors — sodium, blood glucose and blood urea nitrogen (BUN) — to get the calculated osmolality and see how it compares to the normal range of 275–295 mOsm/kg.
mEq/L
mg/dL
mg/dL
Normal range: 275–295 mOsm/kg
- 1
Sodium contribution (2 × Na⁺)
2 × 140 = 280Sodium is doubled to account for accompanying anions (Cl⁻ and HCO₃⁻). - 2
Glucose contribution (Glu ÷ 18)
90 ÷ 18 = 5 - 3
BUN contribution (BUN ÷ 2.8)
14 ÷ 2.8 = 5 - 4
Serum osmolality
280 + 5 + 5 = 290
How does this calculator work?
Estimated serum osmolality = 2×Na (mEq/L) + Glucose (mg/dL)/18 + BUN (mg/dL)/2.8. Normal is 275–295 mOsm/kg. Sodium dominates (roughly 2/3 of the result); elevated glucose or BUN each add smaller but clinically important contributions. Subtract the calculated value from the measured lab result to find the osmol gap.
Formula
How this is calculated
The most widely used clinical formula for estimated serum osmolality is Osm = 2×Na + Glu/18 + BUN/2.8, where sodium (mEq/L) is doubled to account for the major anions that accompany it (chloride and bicarbonate), glucose is divided by 18 to convert mg/dL to mmol/L, and BUN is divided by 2.8 (half the molecular weight of urea nitrogen, 14 × 2 = 28, divided by 10 for the dL-to-L/mg-to-mmol conversion).
The normal range is 275–295 mOsm/kg. Values below 275 (hypo-osmolality) suggest excess free water (e.g. SIADH, dilutional hyponatremia). Values above 295 (hyper-osmolality) point to dehydration, uncontrolled diabetes (high glucose), renal failure (high BUN), or additional unmeasured osmoles such as ethanol.
The osmol gap — the difference between the measured (laboratory) osmolality and this calculated value — is clinically important for detecting toxic alcohols. A gap above 10–15 mOsm/kg suggests unmeasured osmoles. This calculator computes the estimated (calculated) osmolality only; the measured value must come from the laboratory.
Frequently asked questions
Osmolality is measured per kilogram of solvent (mOsm/kg water) — the laboratory value. Osmolarity is per litre of solution (mOsm/L). For dilute solutions like plasma they are nearly identical (~1% difference), and the terms are often used interchangeably in clinical practice.
The osmol gap is the measured osmolality minus the calculated value. A gap above ~10 mOsm/kg suggests unmeasured osmoles, most importantly toxic alcohols (methanol, ethylene glycol, isopropanol) or, less commonly, mannitol or propylene glycol infusions.
Yes. Several formulas exist, including versions that omit BUN (since urea distributes freely across cell membranes and does not contribute to effective/tonicity osmolality) or that include ethanol. The formula used here (2×Na + Glu/18 + BUN/2.8) is the most widely taught and applied version.
Also known as
TG we-Calculate Editorial Team. (2026). Serum Osmolality Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/serum-osmolality-calculator
TG we-Calculate Editorial Team. "Serum Osmolality Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/serum-osmolality-calculator.
TG we-Calculate Editorial Team, "Serum Osmolality Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/serum-osmolality-calculator
@misc{wecalculate_serum_osmolality_calculator, title = {Serum Osmolality Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/serum-osmolality-calculator}}, year = {2026}, note = {TG we-Calculate} }
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