About the Serum Osmolality Calculator
This calculator estimates serum osmolality — the concentration of dissolved particles in plasma — from sodium, glucose and blood urea nitrogen (BUN) using the standard 2 × Na + glucose/18 + BUN/2.8 formula (Smithline–Gardner), with the option to add an ethanol term. Enter a laboratory-measured osmolality and it also returns the osmolal gap, the difference between measured and calculated values.
It is intended for clinicians, pharmacists, nurses and students working through hyponatraemia, hyperglycaemic crises, renal failure or a suspected toxic alcohol ingestion. Normal serum osmolality is about 275–295 mOsm/kg. An osmolal gap above roughly 10 mOsm/kg means unmeasured osmoles are present — most importantly ethanol, methanol, ethylene glycol or isopropanol, but also mannitol, ketones or lactate in some conditions.
You can work in US units (glucose and BUN in mg/dL) or SI units (glucose and urea in mmol/L). The effective osmolality (tonicity), which leaves out urea because it crosses cell membranes freely, is shown too. A normal osmolal gap does not rule out toxic alcohol poisoning, especially late after ingestion.
With the default inputs, the calculated osmolality is 290.6 mOsm/kg. Change any value above to recalculate instantly.
How to use the serum osmolality calculator
- 1Choose US (mg/dL) or SI (mmol/L) units.
- 2Enter sodium, glucose and BUN (or urea).
- 3Add the ethanol level if one was measured.
- 4Enter the lab-measured osmolality to get the osmolal gap.
- 5Read the calculated osmolality, effective osmolality and gap.
Formula and method
Sodium is doubled to account for its accompanying anions (chloride and bicarbonate). Glucose and BUN in mg/dL are converted to mmol/L by dividing by 18 and 2.8 (one tenth of their molecular weights, 180 and 28). In SI units glucose and urea are already in mmol/L and are added directly. Ethanol in mg/dL is divided by 3.7 (or, in SI units, multiplied by 1.25), the empirically derived factor from Purssell et al. (Annals of Emergency Medicine, 2001), whose regression of osmolal gap on ethanol level in emergency patients showed a slightly larger contribution than the molecular weight alone predicts.
The osmolal gap is the measured osmolality minus the calculated value; values up to about 10 mOsm/kg are usually considered normal, though reference ranges vary between labs and formulas. Effective osmolality (tonicity) omits urea and ethanol because they cross cell membranes and do not shift water between compartments.
- Na
- Serum sodium, mmol/L
- BUN
- Blood urea nitrogen, mg/dL
- Osm
- Osmolality, mOsm/kg
Worked examples
Normal labs (US units)
2 × 140 = 280, glucose 100/18 = 5.6 and BUN 14/2.8 = 5.0, giving a calculated osmolality of 290.6 mOsm/kg.
Suspected toxic alcohol with a raised gap
276 + 5.0 + 4.29 + 50/3.7 (13.51) = 298.8 mOsm/kg. With a measured value of 320 the osmolal gap is about 21 mOsm/kg — elevated even after accounting for ethanol, so another osmole should be considered.
Hyperglycaemia in SI units
2 × 132 + 45 + 15 = 324 mOsm/kg. Excluding urea, the effective osmolality is 309 mOsm/kg.
Frequently asked questions
What is a normal serum osmolality?+
Normal serum osmolality is about 275 to 295 mOsm/kg in adults. The calculated value is usually within a few mOsm/kg of the measured value in healthy people.
What is a normal osmolal gap?+
An osmolal gap up to about 10 mOsm/kg is generally considered normal, although the range depends on the laboratory and formula. A gap above 10 suggests unmeasured osmotically active substances.
What causes a high osmolal gap?+
Ethanol is the most common cause. Toxic alcohols (methanol, ethylene glycol, isopropanol), mannitol, glycerol, propylene glycol, and severe ketoacidosis, lactic acidosis or kidney failure can also raise it.
What is the difference between osmolality and osmolarity?+
Osmolality is solute per kilogram of water (mOsm/kg) and is what laboratories measure by freezing-point depression. Osmolarity is per litre of solution. Calculated values are technically osmolarity but are compared with measured osmolality in practice.
Why is effective osmolality different?+
Urea and ethanol pass freely across cell membranes, so they raise total osmolality without pulling water out of cells. Effective osmolality (2 × Na + glucose) better reflects the tonicity that drives water shifts and brain cell volume.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.