Base inputs: 140 / 90 / 14
Sodium 140 mmol/L, glucose 90 mg/dL, and BUN 14 mg/dL produce calculated total 290.0 mOsm/kg and tonicity 285.0 mOsm/kg.
Educational reference only. Not a diagnostic tool. See full disclaimer.
NephrologySerum Osmolality
Calculated Serum Osmolality, Tonicity & Osmolal Gap Calculator
Calculate serum osmolality and effective osmolality (tonicity), with optional measured osmolality, osmolal gap, and separate ethanol assumptions; no toxic-alcohol or HHS diagnosis.
Content updated: View sources
QuickMedCalc is developed and maintained by an independent developer. Medical content is not independently reviewed by a physician.
Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.
About
This page calculates conventional serum osmolality, effective osmolality (tonicity), an optional base osmolal gap, optional ethanol-inclusive totals, and—only when the submitted method supports it—ethanol-adjusted residual gaps. Each output answers a different arithmetic question and the quantities are not interchangeable. [1, 2, 6]
Calculated total uses measured sodium, glucose, and urea or BUN. Tonicity excludes urea and ethanol. Measured osmolality is a laboratory osmometer result; the osmolal gap subtracts the calculated total from that measurement. The two ethanol paths are assumptions applied to one submitted ethanol concentration, not two laboratory measurements. [1, 2, 3, 4]
An osmolal gap is not the concentration of one unmeasured substance and cannot by itself diagnose or exclude methanol, ethylene glycol, or another toxic-alcohol exposure. The page also does not diagnose HHS or generate antidote, dialysis, fluid, insulin, potassium, or other treatment instructions. [8, 9, 10]
Interpretation
| Quantity | Meaning | Unit | Shown when |
|---|---|---|---|
| Calculated total osmolality | Formula estimate including sodium, glucose, and urea | mOsm/kg | Always |
| Effective osmolality / tonicity | Formula estimate excluding urea and ethanol | mOsm/kg | Always |
| Measured osmolality | Laboratory osmometer result | mOsm/kg | User input |
| Base osmolal gap | Measured minus calculated total | mOsm/kg | Measured value supplied |
| Ethanol-inclusive total | Calculated total plus one ethanol assumption | mOsm/kg | Ethanol supplied |
| Ethanol-adjusted residual gap | Measured minus ethanol-inclusive total | mOsm/kg | Measured value, ethanol, and freezing-point method supplied |
Calculated and measured osmolality are not the same result, and tonicity is not the same as total osmolality. A gap can be positive, zero, or negative; it is not an unmeasured-substance concentration. A withheld residual gap does not mean the result is normal. No displayed value automatically generates a diagnosis or treatment. [2, 7, 8]
What this calculator computes
- Sodium, glucose, and BUN or urea produce calculated total osmolality and tonicity.
- Adding measured osmolality produces the signed base osmolal gap.
- Adding ethanol produces two ethanol contributions and two ethanol-inclusive totals, without a primary model.
- Measured osmolality plus ethanol produces residual gaps only when the selected method is freezing-point depression.
- Vapor-pressure, unknown-method, and no-measurement paths explicitly withhold residual gaps rather than silently treating them as zero.
Inputs, units, and same-sample requirement
The sodium field uses the measured value directly; it does not substitute hyperglycemia-adjusted sodium. Glucose accepts mg/dL or mmol/L, BUN mg/dL and urea mmol/L are distinct inputs, measured osmolality is mOsm/kg, and ethanol is mg/dL. Values must represent the same specimen or a reasonably paired clinical time point because these analytes can change independently. Changing an input or unit clears confirmation and previous results so an old number is not silently reinterpreted. [1, 2]
Calculated total versus tonicity
Urea contributes to conventional total osmolality but is usually not a sustained effective osmole across cell membranes, so the tonicity expression excludes it. This implementation also excludes ethanol from tonicity. Total osmolality and tonicity therefore answer different questions and should not be relabeled as one another. [9, 10]
Measured osmolality and the osmolal gap
Measured osmolality comes from an osmometer; calculated total comes from a formula. Their difference is the base osmolal gap. Formula choice, analyzer platform, measurement uncertainty, population, and baseline unmeasured solutes can all affect it, so positive, zero, and negative values are preserved without a dynamic normal/abnormal label. [2, 7]
Why the measurement method matters
Freezing-point depression can reflect volatile solutes, whereas vapor-pressure osmometry may not capture volatile alcohols. When the method is unknown, the comparison is also unsafe. The calculator therefore reports ethanol-adjusted residual gaps only for a submitted freezing-point result. Withholding the residual is a transparent software boundary; it does not mean ethanol or another toxic alcohol is absent. [6]
Why two ethanol assumptions are shown
The ÷3.7 path is the simplified Purssell clinical regression; the ÷4.6 path is the molecular-weight assumption supported by Nguyen’s experiment. In a 2025 prospective healthy-volunteer study, ÷4.6 tracked each participant’s baseline-adjusted gap better than ÷3.7, but baseline and measurement variation remained. Different study designs do not establish one universal value for every person, so both estimates are shown and neither is automatically primary. [3, 4, 5]
Worked examples
Measured gap: 295
The same base inputs plus measured osmolality 295 mOsm/kg by freezing point produce a base gap of 5.0 mOsm/kg.
Hyperglycemia: 140 / 600 / 28
Calculated total is about 323.3 mOsm/kg and tonicity about 313.3 mOsm/kg. Those numbers alone do not diagnose HHS.
Equivalent conventional and SI units
Glucose 90 mg/dL equals 5 mmol/L for this formula path; BUN 14 mg/dL equals urea 5 mmol/L. Both paths produce the same raw results.
Ethanol: 230 mg/dL
With base inputs 140 / 90 / 14, measured osmolality 350, and freezing-point method, the base total is 290.0 and gap 60.0. The ÷4.6 contribution is 50.0, inclusive total 340.0, and residual 10.0. The ÷3.7 contribution is about 62.2, inclusive total about 352.2, and residual about −2.2 mOsm/kg.
These examples audit the frozen implementation’s formula, unit, optional-input, method, and display paths. They are not patient interpretation or treatment advice.
Osmolal gap and toxic-alcohol limits
An elevated gap can be a clue but is nonspecific. Baseline gap, formula, measurement method, and the metabolic stage after exposure all affect interpretation; as a parent alcohol is metabolized, its osmolal contribution may fall while an anion gap changes. A normal or small gap cannot exclude exposure, and an elevated gap cannot identify one substance or its concentration. This page does not run a poisoning diagnosis or treatment pathway. [7, 8]
Hyperglycemic-crisis context
Current adult HHS criteria include hyperosmolarity defined as calculated effective osmolality above 300 mOsm/kg or total osmolality above 320 mOsm/kg. HHS diagnosis requires the rest of the published criteria as well; this page does not collect them, does not label a submitted result as HHS, and does not generate fluid, insulin, potassium, or monitoring instructions. [9, 10]
When results may be less reliable
- Inputs are not from the same specimen or a reasonably paired time point.
- Sodium, glucose, or urea is changing rapidly.
- Infusion, dialysis, diuresis, or another major fluid shift is underway.
- The measurement method is unknown, or the analyzer or laboratory formula differs.
- Ethanol and other unmeasured osmoles coexist.
- The person’s baseline osmolal gap differs from an assumed reference.
- Testing occurs at a different metabolic stage after an exposure.
- A calculated estimate is mistaken for a laboratory measurement.
- A gap is treated as the concentration of one substance.
These are static interpretation limits, not individualized diagnostic or treatment instructions. [2, 5, 6, 7, 8]
References
- Worthley LI, Guerin M, Pain RW. For calculating osmolality, the simplest formula is the best. Anaesth Intensive Care. 1987;15(2):199–202. PMID 3605570. DOI 10.1177/0310057X8701500214.
- Choy KW, et al. Harmonisation of Osmolal Gap — Can We Use a Common Formula? Clin Biochem Rev. 2016;37(3):113–119. PMID 27872505. PMCID PMC5111243.
- Purssell RA, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653–659. PMID 11719745. DOI 10.1067/mem.2001.119455.
- Nguyen MK, et al. Is the Osmolal Concentration of Ethanol Greater Than Its Molar Concentration? Front Med. 2020;6:306. PMID 31970159. PMCID PMC6960184. DOI 10.3389/fmed.2019.00306.
- Marino R, et al. Ethanol and the Limitations of the Osmol Gap. Ann Emerg Med. 2025;86(3):257–261. PMID 39864007. DOI 10.1016/j.annemergmed.2024.12.022.
- Draviam EJ, Custer EM, Schoen I. Vapor pressure and freezing point osmolality measurements applied to a volatile screen. Am J Clin Pathol. 1984;82(6):706–709. PMID 6507382. DOI 10.1093/ajcp/82.6.706.
- Skaaland H, et al. Reference values for osmolal gap in healthy subjects and in medical inpatients. Scand J Clin Lab Invest. 2020;80(1):1–5. PMID 31809199. DOI 10.1080/00365513.2019.1672086.
- Kraut JA, Xing SX. Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. 2011;58(3):480–484. PMID 21794966.
- Umpierrez GE, et al. Hyperglycemic Crises in Adults With Diabetes: A Consensus Report. Diabetes Care. 2024;47(8):1257–1275. PMID 39052901. DOI 10.2337/dci24-0032.
- American Diabetes Association. Diabetes Care in the Hospital: Standards of Care in Diabetes—2026.
FAQ
Osmolality is expressed per kilogram of solvent and is the laboratory terminology used here. Osmolarity is per liter of solution; the terms should not be silently interchanged in a laboratory result.
Sources: [2]
No. A gap is nonspecific and a low or negative gap does not exclude exposure. This page does not diagnose toxic alcohols or choose a treatment.
Sources: [8]
As a parent alcohol is metabolized, its osmolal contribution can fall while organic-acid contribution to an anion gap can rise. That time course is context, not a diagnostic rule in this calculator.
Sources: [8]
Freezing-point depression can reflect volatile solutes. Vapor-pressure osmometry may not capture volatile alcohols, so the page withholds ethanol-adjusted residual gaps for vapor-pressure measurements.
Sources: [6]
Sodium, glucose, urea/BUN, ethanol, and measured osmolality can change over time. Combining unrelated values can create a difference that never represented one clinical state.
Sources: [2]
The page shows conventional total, tonicity, and any base osmolal gap without ethanol models.
Sources: [2]
The page cannot safely compare an ethanol-inclusive total with a measurement that may not capture volatile alcohols, or whose method is unknown.
Sources: [6]
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Estimate a static positive free water deficit for hypernatremia when current sodium is >145 mmol/L or mEq/L, using an explicitly selected lower target, kg or lb weight, and an explicit TBW coefficient, with estimated TBW and results in L and mL—not an infusion volume or treatment output.
Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.