Serum Osmolality Calculator
Calculate serum osmolality, effective osmolality (tonicity), osmol gap, corrected sodium, and a rough free-water estimate in one interpretation worksheet.
Plasma Osmolality Calculator
Calculate plasma osmolality, effective osmolality (tonicity), osmol gap, corrected sodium for hyperglycemia, and a simple free-water estimate.
⚠️ Medical Disclaimer: Osmolality assessment requires clinical correlation. Always verify with measured osmolality in critical scenarios.
Planning notes, formulas, and examples
About the Plasma Osmolality Calculator
Plasma osmolality summarizes the concentration of dissolved solutes in blood and is usually reviewed together with sodium, glucose, and urea results. It is useful for organizing hypo-osmolar and hyperosmolar states, comparing measured versus calculated osmolality, and checking whether hyperglycemia is distorting the measured sodium.
This page calculates total osmolality from sodium, glucose, and BUN, then shows effective osmolality (tonicity), the osmol gap when a measured value is available, corrected sodium for hyperglycemia, and a simple free-water estimate for hypernatremia. It also includes an osmol-gap differential table so the measured-versus-calculated difference can be reviewed in context.
The result is an interpretation aid rather than a poisoning diagnosis or IV-replacement protocol. Measured osmolality, acid-base status, timing of ingestion, renal function, and the broader clinical picture still determine how an elevated osmol gap or hyperosmolar state should be interpreted.
When This Page Helps
Plasma osmolality is easiest to interpret when the calculated value, effective tonicity, and osmol gap are reviewed together. This calculator keeps the sodium, glucose, and BUN inputs linked to the measured osmolality and free-water estimate so the result can be used for hyponatremia, hypernatremia, DKA, or toxic alcohol workups without separating the related numbers.
How to Use the Inputs
- Enter sodium (required), glucose, and BUN values
- Select mg/dL or mmol/L for glucose and BUN
- Optionally enter measured osmolality to calculate the osmol gap
- Enter ethanol level if alcohol ingestion is suspected
- Review calculated and effective osmolality, corrected sodium, and osmol gap
- Use the differential diagnosis table to interpret elevated osmol gaps
- Use presets to explore DKA, hypernatremia, and toxic ingestion scenarios
Formula used
Calculated Osm = 2 × Na + Glucose/18 + BUN/2.8 (all in mg/dL). Effective Osm = 2 × Na + Glucose/18. Osmol Gap = Measured Osm − Calculated Osm. Corrected Na = Na + 1.6 × ((Glucose − 100) / 100). Free Water Deficit = 0.6 × Weight × (Na/140 − 1).Example Calculation
Result: Calculated Osm = 290 mOsm/kg, Osmol Gap = 30, Significantly elevated — investigate toxic alcohols
With Na 140, glucose 90, and BUN 14, calculated osmolality = 2×140 + 90/18 + 14/2.8 = 290. A measured osmolality of 320 gives an osmol gap of 30, which is clearly elevated and should be reviewed with the acid-base pattern, ingestion history, and the rest of the laboratory workup.
Tips & Best Practices
- Always compare calculated and measured osmolality — the gap is more informative than either value alone
- In DKA, correct sodium for hyperglycemia before assessing true sodium status
- An osmol gap can normalize as toxic alcohols are metabolized to their acid metabolites — check both osmol gap and anion gap
- Ethanol is the most common cause of elevated osmol gap in the ED
- Free water replacement rate for hypernatremia: correct Na ≤10–12 mEq/L per 24 hours to avoid cerebral edema
- Isopropanol causes elevated osmol gap WITHOUT anion gap metabolic acidosis (metabolized to acetone)
Total Osmolality Versus Tonicity
Calculated osmolality includes urea, but effective osmolality excludes it because urea does not meaningfully hold water in the extracellular space. That distinction matters when a patient has a high lab osmolality without the neurologic changes expected from true hypertonicity.
When the Osmol Gap Matters
An elevated osmol gap suggests unmeasured osmoles such as toxic alcohols, mannitol, or ethanol. A normal gap does not fully exclude toxic alcohol ingestion if the exposure is delayed, so the gap should be read together with the anion gap, acid-base status, and clinical history.
Hyperglycemia and Hypernatremia
Hyperglycemia lowers the measured sodium by shifting water into the extracellular space, so corrected sodium helps determine the true tonicity problem. In hypernatremia, free-water deficit estimates are most useful when they are followed by a controlled correction plan rather than a one-time replacement number.
Sources & Methodology
Last updated:
Methodology
This page calculates total osmolality from sodium, glucose, and BUN, then shows effective osmolality, corrected sodium for hyperglycemia, the osmol gap when a measured value is entered, and a simple free-water estimate for hypernatremia. It is meant to keep the related osmolality numbers together so the user can compare measured and calculated values in one place.
The result is an interpretation aid, not a poisoning diagnosis or a fluid-prescription engine. Osmol-gap interpretation depends on timing, acid-base status, renal function, and the broader clinical picture, and the free-water number is only a rough estimate rather than a stand-alone replacement plan.
Sources
- Clinical Physiology of Acid-Base and Electrolyte Disorders (McGraw-Hill) — Standard reference for osmolality, tonicity, corrected sodium, and water-balance interpretation.
- The osmolal gap in the diagnosis of methanol or ethylene glycol ingestion (Annals of Emergency Medicine) — Reference for osmol-gap use and limitations in suspected toxic alcohol exposure.
Frequently Asked Questions
The normal osmol gap ranges from −10 to +10 mOsm/kg. Some sources cite a mean of approximately +2. Gaps consistently above +10 warrant clinical evaluation for unmeasured osmoles.
An elevated osmol gap can appear with ethanol, toxic alcohols, mannitol, contrast agents, propylene glycol, and some severe metabolic states. The gap is a clue, not a diagnosis, so it should be read together with timing, acid-base status, and the rest of the clinical workup.
BUN (urea) freely crosses cell membranes along its concentration gradient. It does not create an osmotic gradient between intracellular and extracellular fluid. Therefore, it does not contribute to "effective" osmolality or tonicity, which determines water movement between compartments.
Corrected Na = Measured Na + 1.6 × ((Glucose − 100) / 100). For every 100 mg/dL glucose above normal, sodium decreases by approximately 1.6 mEq/L due to osmotic water shift from ICF to ECF. This is dilutional, not true hyponatremia.
Yes. A significantly negative osmol gap (<−10) usually indicates laboratory artifact, most commonly due to hypertriglyceridemia or hyperproteinemia causing pseudohyponatremia in some assay methods.
The page uses a simple body-water estimate to show how much free water would theoretically bring sodium back toward normal. It is best treated as a planning number rather than a stand-alone fluid prescription, because true replacement depends on chronicity, ongoing losses, weight, and the clinical setting.
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