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Calculate estimated serum osmolality from routine chemistries using 2×Na + glucose/18 + BUN/2.8 (US) or 2×Na + glucose + urea (SI, mmol/L). Optionally enter measured osmolality to compute the osmolar gap—useful in toxic alcohol and hyponatremia teaching. Pair with our anion gap calculator. Educational—not for treatment decisions.
Last updated: June 5, 2026
Does not include ethanol, mannitol, or toxic alcohol terms. Suspected ingestion: contact poison control. Not for dialysis, hypertonic saline, or IV fluid prescribing.
Unit system
Conventional US formula
2 × Na (mEq/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8 → estimated mOsm/kg
Estimated osmolality
290.9 mOsm/kg
Typical measured serum osmolality is often near 275–295 mOsm/kg; this is an estimate from a few solutes only.
Limitations
Does not include ethanol, ketones, mannitol, or other osmoles. Some labs report slightly different reference formulas. Not for diagnosis or treatment decisions.
Default — Na 140, glu 100, BUN 15
290.9 mOsm/kg
Typical measured range ~275–295
Hyperglycemia — glucose 400 mg/dL
307.6 mOsm/kg
+16.6 from glucose vs default
Measured 330 — elevated gap
+39.1 gap
Calc 290.9 mOsm/kg
Uremia — BUN 80 mg/dL
314.1 mOsm/kg
Elevated nitrogen term
SI equivalent (Na 140, glucose 5.6, urea 5.4 mmol/L) → 291 mOsm/kg — matches US default when units converted correctly.
Choose one unit system and apply consistently—mixing mg/dL and mmol/L invalidates the result.
| Mode | Formula | Units | Notes |
|---|---|---|---|
| US (conventional) | 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 | Na mEq/L; glucose mg/dL; BUN mg/dL | Most common teaching formula in U.S. curricula and boards |
| SI / international | 2 × Na + glucose + urea | Na, glucose, urea all mmol/L | Direct millimolar sum once units are aligned |
| Analyte | US | SI | Conversion |
|---|---|---|---|
| Glucose | mg/dL | mmol/L | ÷ 18 (or × 0.0555) |
| BUN → urea | BUN mg/dL | Urea mmol/L | BUN ÷ 2.8 ≈ urea mmol/L (approx.) |
| Sodium | mEq/L | mmol/L | Numerically identical for monovalent Na⁺ |
| Parameter | Typical | Note |
|---|---|---|
| Measured serum osmolality | 275–295 mOsm/kg | Lab-specific; pediatric and assay method vary |
| Calculated osmolality (teaching) | Often within ~±10 of measured | When only Na, glucose, urea included |
| Osmolar gap | Roughly −10 to +10 mOsm/kg | Institution thresholds differ; context-dependent |
| Gap | Meaning | Action |
|---|---|---|
| ≤10 mOsm/kg | Broadly consistent | Reassuring if chemistries and timing align; still correlate clinically |
| >10 mOsm/kg | Elevated — unmeasured osmoles possible | Consider ethanol, toxic alcohols, mannitol, contrast, immunoglobulins |
| <−10 mOsm/kg | Negative gap | Review assay method, hyperlipidemia/hyperproteinemia (pseudohyponatremia), lab error |
| Cause | Detail |
|---|---|
| Ethanol | Adds osmoles; often requires specific ethanol term or nomogram beyond basic Na/glucose/BUN |
| Methanol / ethylene glycol / isopropanol | Classic toxic alcohol workup with high anion gap acidosis as metabolism progresses |
| Mannitol / contrast / propylene glycol | Iatrogenic osmoles—medication and procedure review |
| Severe hyperglycemia / uremia | Usually captured by formula; very high values still raise calculated osm |
| Lab timing mismatch | Measured and chemistry drawn at different times—repeat paired specimens |
| Scenario | Role of calculated osm |
|---|---|
| Hyponatremia evaluation | Effective osmolality distinguishes hypotonic vs isotonic/hypertonic hyponatremia |
| Hypernatremia / water deficit | Elevated calculated osm supports hypertonic state when paired with clinical volume assessment |
| Toxic alcohol ingestion | Early high osmolar gap with later high anion gap acidosis is a classic pattern |
| Diabetic hyperosmolar state | Markedly elevated glucose contribution to calculated osmolality |
Result: 290.9 mOsm/kg — within typical measured serum range
Gap +39.1 mOsm/kg — prompts toxic alcohol/ethanol review, not formula error alone
An osmolar gap over about 10 mOsm/kg suggests unaccounted osmoles (e.g. ethanol, methanol, ethylene glycol, mannitol, or other solutes). Correlate clinically and with toxicology as indicated.
Glucose term rises from 5.6 to 22.2 mOsm/kg (+16.6), pushing calculated osm to 307.6 mOsm/kg—illustrating hyperosmolar stress in severe hyperglycemia teaching.
Educational use only. This calculator does not replace poison control (U.S. 1-800-222-1222), emergency evaluation, or laboratory reference methods. Pair calculated values with measured osmolality from your institution when clinical decisions depend on tonicity.
Help students and colleagues review fluid, electrolyte, and osmolar gap concepts
Suggested hashtags: #Osmolality #OsmolarGap #LabMedicine #Nephrology #MedicalCalculator