Osmolality Calculator
Calculate serum osmolality based on sodium, glucose, and BUN values to assess fluid and electrolyte balance in clinical settings.
Calculate Serum Osmolality
Enter laboratory values to calculate patient's serum osmolality
Calculation Results
Based on validated clinical formulas used in healthcare settings worldwide
Use this calculator on any device at the bedside or in your clinic
Get immediate osmolality calculations and clinical interpretations
Formula Used
Where:
• Na+ is sodium concentration in mEq/L
• Glucose is in mg/dL
• BUN (Blood Urea Nitrogen) is in mg/dL
Understanding Serum Osmolality
Serum osmolality is a measure of the body's electrolyte-water balance. It represents the concentration of osmotically active particles in the blood serum, measured in milliosmoles per kilogram of water (mOsm/kg).
Normal Range
The normal serum osmolality range is 275-295 mOsm/kg of water. Values outside this range may indicate various clinical conditions.
| Osmolality Value | Clinical Significance |
|---|---|
| 275-295 mOsm/kg | Normal Range |
| < 275 mOsm/kg | Hypo-osmolality |
| > 295 mOsm/kg | Hyperosmolality |
Clinical Applications
- Evaluating hydration status and fluid balance
- Detecting hyponatremia and hypernatremia
- Identifying osmolar gap in cases of intoxication
- Assessing diabetic emergencies (DKA, HHS)
- Monitoring renal function and conditions
Causes of High Serum Osmolality (>295 mOsm/kg)
- Dehydration
- Diabetes mellitus (particularly hyperglycemic states)
- Diabetes insipidus
- Severe renal failure
- Alcohol intoxication
- Mannitol administration
Causes of Low Serum Osmolality (<275 mOsm/kg)
- Syndrome of inappropriate antidiuretic hormone (SIADH)
- Hyponatremia
- Excessive water intake
- Freshwater drowning
- Congestive heart failure
- Liver cirrhosis
References
- Rasouli M. (2016). Basic concepts and practical equations on osmolality: Biochemical approach. Clinical Biochemistry, 49(12), 936-941.
- MDCalc. Serum Osmolality/Osmolarity Calculator.
- Balal M, Shadow V, Chaudhuri A. (2023). Osmometer. StatPearls.
- Fazekas AS, Funk GC, Klobassa DS, et al. (2013). Evaluation of 36 formulas for calculating plasma osmolality. Intensive Care Med, 39(2), 302-8.
