Plasma Osmolality Calculator — Serum Osmolarity Formula
Compute estimated plasma osmolality from serum sodium, glucose and BUN using the standard clinical formula, identify hypo- or hyper-osmolar states, and optionally calculate the osmol gap if a measured value is available.
mEq/L
mg/dL
mg/dL
mOsm/kg
Normal (275–295 mOsm/kg)
- 1
Sodium contribution (2 × Na⁺)
2 × 140 = 280 - 2
Glucose contribution (÷ 18)
90 ÷ 18 = 5Dividing mg/dL by 18 converts glucose to mmol/L (mOsm/kg). - 3
BUN contribution (÷ 2.8)
15 ÷ 2.8 = 5.36 - 4
Plasma osmolality (sum)
280 + 5 + 5.36 = 290.4
How does this calculator work?
Plasma osmolality = 2 × Na⁺ + Glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. Normal is 275–295 mOsm/kg. An osmol gap (measured − calculated) above 10 mOsm/kg points to unmeasured osmoles such as toxic alcohols. This calculator is educational — clinical decisions require physician review.
Formula
How this is calculated
Plasma osmolality reflects the concentration of solutes dissolved in blood plasma, primarily sodium and its associated anions, glucose, and urea. The Smithline–Gardner formula (1976) estimates it from three routine laboratory values: serum sodium in mEq/L (multiplied by 2 to account for accompanying anions such as chloride and bicarbonate), serum glucose in mg/dL divided by 18 (converting to mmol/L), and blood urea nitrogen (BUN) in mg/dL divided by 2.8 (converting to mmol/L). Summing these three components gives the calculated osmolality in milliosmoles per kilogram of water (mOsm/kg H₂O).
The normal range for plasma osmolality in adults is approximately 275–295 mOsm/kg H₂O. Values below 275 (hypo-osmolar) suggest excess free water relative to solutes — common causes include syndrome of inappropriate antidiuretic hormone secretion (SIADH), excessive water intake, or hypotonic fluid resuscitation. Values above 295 (hyper-osmolar) suggest a relative water deficit — common causes include dehydration, uncontrolled hyperglycaemia (as in diabetic hyperosmolar state), or severe uraemia.
The osmol gap is the difference between the directly measured osmolality (from a laboratory freezing-point osmometer) and the calculated value. A gap above 10 mOsm/kg indicates the presence of unmeasured osmoles — most importantly toxic alcohols such as ethanol, methanol, ethylene glycol, or isopropanol. This calculator is educational; clinical decisions require correlation with the full clinical picture and physician review.
Frequently asked questions
The normal range for serum/plasma osmolality in adults is approximately 275–295 mOsm/kg H₂O. Values outside this range indicate hypo-osmolar or hyper-osmolar states that typically require clinical evaluation.
An osmol gap above 10 mOsm/kg suggests the presence of unmeasured osmoles in the blood. The most important clinical cause is toxic alcohol ingestion (ethanol, methanol, ethylene glycol, isopropanol). In the appropriate clinical context it prompts urgent evaluation and treatment.
Sodium is the dominant cation in plasma and must be electrically balanced by anions (primarily chloride and bicarbonate). Multiplying sodium by 2 approximately accounts for all of these paired ions and their osmotic contribution.
Also known as
TG we-Calculate Editorial Team. (2026). Plasma Osmolality Calculator — Serum Osmolarity Formula [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/plasma-osmolality-calculator
TG we-Calculate Editorial Team. "Plasma Osmolality Calculator — Serum Osmolarity Formula." TG we-Calculate. 2026. https://we-calculate.com/calculator/plasma-osmolality-calculator.
TG we-Calculate Editorial Team, "Plasma Osmolality Calculator — Serum Osmolarity Formula," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/plasma-osmolality-calculator
@misc{wecalculate_plasma_osmolality_calculator, title = {Plasma Osmolality Calculator — Serum Osmolarity Formula}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/plasma-osmolality-calculator}}, year = {2026}, note = {TG we-Calculate} }
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