Serum osmolality calculator with osmolal gap
For healthcare professionals and students. Check results against your local protocols and clinical judgement; this tool does not give medical advice.
This osmolality calculator estimates serum osmolality from sodium, glucose and urea — in SI units (mmol/L) or US units (glucose and BUN in mg/dL) — and, when you enter a laboratory-measured osmolality, the osmolal gap. An optional ethanol term uses the coefficient derived by Purssell and colleagues, so the gap left over is the part ethanol cannot explain. You can switch to the Dorwart–Chalmers equation to compare.
A second mode works out the theoretical osmolarity of an IV fluid from its sodium chloride, glucose and added potassium chloride, which answers a common “osmolarity calculator” question about normal saline, dextrose and mixtures. Everything runs in your browser.
How to use the Serum Osmolality Calculator
- Choose Serum osmolality and gap or IV fluid osmolarity.
- For serum, enter sodium, glucose (mg/dL or mmol/L) and BUN or urea (mg/dL BUN or mmol/L urea).
- Add ethanol if it was measured, and the measured osmolality from the laboratory to get the osmolal gap.
- For a fluid, enter the % w/v of sodium chloride and glucose, how the glucose is stated on the label, and any potassium chloride in mmol/L.
- Read the result, the working and the notes.
Serum Osmolality Calculator features
SI or US units
Glucose in mg/dL or mmol/L, BUN in mg/dL or urea in mmol/L, using 18.016 and 2.801 rather than rounded divisors.
Osmolal gap with ethanol
Measured minus calculated osmolality, with ethanol ÷ 3.7 (mg/dL) or × 1.25 (mmol/L) included when entered.
Two equations
The standard 2 Na + glucose + urea, or Dorwart and Chalmers’ 1.86 Na + glucose + urea + 9.
IV fluid osmolarity
Sodium chloride, glucose (monohydrate or anhydrous labelling) and potassium chloride added up per litre.
Sense checks
Flags unit mix-ups, a gap above 10 mOsm/kg, a large negative gap and the limits of a normal gap.
The serum osmolality formula
Calculated osmolality = 2 × Na + glucose + urea with all three in mmol/L, or in US units 2 × Na + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8 — the divisors are the molar masses of glucose (180.16 g/mol) and of the two nitrogen atoms in urea (28.01 g/mol), divided by 10. Sodium is doubled to account for its accompanying anions.
Hand-calculated examples used in the self-test: sodium 140, glucose 5 and urea 5 mmol/L give 290 mOsm/kg; in US units, sodium 140, glucose 90 mg/dL and BUN 14 mg/dL give 280 + 5.0 + 5.0 = 290.0. With a measured osmolality of 300, the gap is 10.0 mOsm/kg.
Dorwart WV and Chalmers L, Clin Chem 1975;21(2):190–194, compared 13 published methods in 715 patient sera and found 1.86 × Na + glucose/18 + BUN/2.8 + 9 the most accurate; the tool offers it as an alternative.
Osmolal gap and ethanol
The osmolal gap is measured minus calculated osmolality. It rises when unmeasured osmotically active substances are present — most often ethanol, but also methanol, ethylene glycol, isopropanol, mannitol and others. Purssell RA et al., Ann Emerg Med 2001;38(6):653–659, derived and validated the ethanol contribution as ethanol (mg/dL) ÷ 3.7, or 1.25 × ethanol (mmol/L); in their validation group the residual gap averaged 0.84 with a standard deviation of 5.65.
A gap above about 10 mOsm/kg is commonly treated as raised, but the normal range is wide and depends on the laboratory and the equation. A normal gap does not rule out toxic alcohol poisoning, because the parent alcohol is converted to acids over time. Interpret it with the anion gap, the history and your toxicology service.
Osmolarity of common IV fluids
Theoretical osmolarity assumes complete dissociation: sodium chloride gives two particles per molecule (58.44 g/mol), glucose one. Labels differ in how glucose is stated: US products are usually labelled as dextrose monohydrate (198.17 g/mol), so 5% dextrose is about 252 mOsm/L; UK and EU labels state anhydrous glucose (180.16 g/mol), giving about 278 mOsm/L. Glucose is metabolised after infusion, so glucose-containing fluids act as free water — relevant to the free water deficit calculator and the maintenance fluid calculator.
| Fluid | Theoretical osmolarity (mOsm/L) |
|---|---|
| 0.9% sodium chloride | 308 |
| 0.45% sodium chloride | 154 |
| 5% glucose (monohydrate / anhydrous) | 252 / 278 |
| 0.45% sodium chloride + 5% glucose (monohydrate) | 406 |
| 0.9% sodium chloride + 20 mmol/L KCl | 348 |
Serum Osmolality Calculator FAQ
How do you calculate serum osmolality?
2 × sodium + glucose + urea in mmol/L, or 2 × Na + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8 in US units. The osmolality calculator above shows the working in either unit system.
What is a normal osmolal gap?
Commonly under about 10 mOsm/kg, but the range is wide. A normal gap does not exclude toxic alcohol ingestion.
How does ethanol affect the osmolal gap?
Each 100 mg/dL of ethanol adds about 27 mOsm/kg (ethanol ÷ 3.7). Enter it to see how much of the gap ethanol explains.
What is the difference between osmolality and osmolarity?
Osmolality is per kilogram of water (mOsm/kg) and is what the laboratory measures; osmolarity is per litre of solution (mOsm/L) and is what fluid labels state.
Why is 0.9% saline 308 mOsm/L but called isotonic?
308 is the theoretical value. Sodium and chloride do not act fully independently, so its measured osmolality is about 286 mOsm/kg, close to plasma.
Can I use this for dogs or cats?
The chemistry is the same, but reference ranges for animals differ; use your veterinary laboratory’s ranges and your vet’s interpretation.