Calculated Serum Osmolality (SI Units)
This calculator uses the SI formula with all inputs in mmol/L:
Calculated osmolality (mOsm/kg) ≈ 2 × [Na⁺] + [glucose] + [urea]
This is an approximation for clinical use. Always interpret results in conjunction with the measured osmolality from the laboratory and the clinical context.
Normal serum osmolality is typically about 275–295 mOsm/kg. An osmolal gap (measured – calculated) > 10–15 mOsm/kg may suggest the presence of unmeasured osmoles (e.g. ethanol, methanol, ethylene glycol, mannitol), and should prompt urgent senior review.
| Indication / Abnormality | Definition | Interpretation | Clinical Implications |
|---|---|---|---|
| Serum osmolality | Concentration of osmotically active particles in plasma, expressed in mOsm/kg. | Normal range ≈ 275–295 mOsm/kg (check your lab reference). | Helps assess water balance and investigate dysnatraemia, toxic alcohol ingestion and some causes of altered mental status. |
| High osmolality | Elevated concentration of solutes in the blood. | May indicate hypernatraemia, severe hyperglycaemia, dehydration or presence of unmeasured osmoles (e.g. ethanol, methanol, mannitol). | Requires evaluation of volume status, sodium, glucose and possible toxins. Management may include fluid resuscitation and specific treatment of the underlying cause (e.g. insulin, antidotes, dialysis). |
| Low osmolality | Reduced concentration of solutes in the blood. | Often seen in true hypotonic hyponatraemia (especially if sodium is low and glucose is normal). | Requires careful assessment of volume status (hypovolaemic / euvolaemic / hypervolaemic hyponatraemia) and slow, controlled correction to avoid osmotic demyelination. |
| Osmolal gap |
Difference between measured and calculated osmolality:
Gap = measured – calculated |
A gap > ~10–15 mOsm/kg suggests unmeasured osmoles (e.g. toxic alcohols, mannitol, radiocontrast), lab variation or calculation limitations. | In the ED, a high gap in a sick patient (especially with acidosis and AKI) should prompt urgent senior review and consideration of toxicology workup and nephrology input. |