Effective Serum Osmolality (Tonicity)
Effective serum osmolality (tonicity) = 2 x sodium (mEq/L) + glucose (mg/dL) / 18.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Serum sodium
- 140 mEq/L
- Serum glucose
- 900 mg/dL
Result: Effective osmolality 330 mOsm/kg — markedly elevated (HHS diagnostic range).
The tool opens with these values already filled in. Replace them with your own.
What you enter
- Serum sodium (mEq/L)
- Serum glucose (mg/dL)
How this is calculated
- Formula
- Effective osmolality = 2 x sodium (mEq/L) + glucose (mg/dL) / 18. Urea is excluded (this is tonicity, not total osmolality).
- Range
- Reference ~275-295 mOsm/kg; > 320 mOsm/kg is a diagnostic criterion for the hyperosmolar hyperglycemic state (HHS).
Kitabchi AE, Umpierrez GE, Miles JM, Fisher JN. Hyperglycemic crises in adult patients with diabetes. Diabetes Care. 2009;32(7):1335-1343. (Effective osmolality per the ADA HHS criteria.) Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.