Acid-Base

Serum Osmolality Calculator

Calculated serum osmolality from sodium, glucose and BUN; compare it with the measured value to find an osmolar gap.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

(2 * Na) + (Glucose / 18) + (BUN / 2.8)

Reference: Normal 285–295

How it is used

• Normal serum osmolality is 285–295 mOsm/kg.
• Measured minus calculated is the osmolar gap, normally under 10; a higher gap suggests an unmeasured osmole such as methanol or ethylene glycol (Berend 2014).
• Ethanol raises the measured value; use the Osmolar Gap calculator to account for it.
• Urea crosses cell membranes, so it raises osmolality without moving water; sodium and glucose set the effective osmolality (tonicity).

Same formula as the app. Open it in the web app.

Related one-pagers

• Endocrine EmergenciesSerum Na, osmolality · Low, <275 · Low, <275 · High, >295
• Electrolyte ImbalancesSend urine osmolality and Na with the serum sample, before saline or diuretics change them. The European guideline (2014) works in this order: serum osmolality under 275 mOsm/kg confirms hypotonicity; urine osmolality 100 or less means intake exceeds maximal excretion (polydipsia, beer potomania); above 100, urine Na 30 mEq/L or less means low effective arterial volume. Volume status comes last because bedside assessment is unreliable.
• DKA and HHSHHS mortality is 5–10 times that of DKA (2024 consensus), and treatment risk lies in moving water too fast. Brain cells adapt to days of hypertonicity by making intracellular osmoles; if serum osmolality falls faster than these clear, water enters the brain, and fast sodium shifts risk osmotic demyelination. Mental impairment is more common above 330 mOsm/kg (JBDS 2023); stupor with effective osmolality below 320 calls for another explanation (2009 consensus).
• ABG Interpretation in 6 Steps

Related calculators

• Albumin-corrected Anion Gap
• Delta Ratio
• Osmolar Gap
• Winters' Formula

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.