Serum Osmolality Calculator
Calculate serum osmolality from sodium, BUN, and glucose to identify hyper- or hypoosmolar conditions.
Input Values
Enter lab values to calculate serum osmolality.
Normal: 136–145 mEq/L
Normal: 7–20 mg/dL
Normal fasting: 70–100 mg/dL
Enter to compute the osmolal gap.
Results
Enter values and press Calculate.
Calculated Osmolality
—
mOsm/kg
| Component | Formula | Value |
|---|---|---|
| 2 × Sodium | 2 × Na | — |
| BUN contribution | BUN / 2.8 | — |
| Glucose contribution | Glucose / 18 | — |
Effective Osmolality (Tonicity)
2×Na + Glucose/18 — excludes BUN
—
Osmolal Gap
Measured − Calculated (normal < 10)
—
Reference Ranges
Hypoosmolality
< 275 mOsm/kg
Normal
275–295 mOsm/kg
Hyperosmolality
> 295 mOsm/kg
Severe hyperosmolality
> 320 mOsm/kg
Osmolal gap (normal)
< 10 mOsm/kg
For educational and screening purposes only. Clinical decisions require measured laboratory values and physician evaluation.
Summary
Calculate serum osmolality from sodium, BUN, and glucose to identify hyper- or hypoosmolar conditions.
How it works
- Enter the serum sodium (Na) in mEq/L.
- Enter the blood urea nitrogen (BUN) in mg/dL.
- Enter the serum glucose in mg/dL.
- The calculator applies 2×Na + BUN/2.8 + Glucose/18.
- Optionally enter a measured osmolality to compute the osmolal gap.
- Review the result against the normal range (275–295 mOsm/kg).
Use cases
- Identify hyperosmolar hyperglycemic state (HHS) in diabetic patients.
- Evaluate the osmolal gap to detect toxic alcohol ingestion.
- Screen for SIADH or other causes of hyponatremia.
- Monitor fluid and electrolyte status in critically ill patients.
- Assess hydration status in perioperative care.
- Support diagnosis of diabetes insipidus.
- Calculate effective osmolality (tonicity) excluding BUN.
Frequently Asked Questions
Last updated: 2026-06-14 ·
Reviewed by Nham Vu