Free Clinical Calculator
Osmolality Calculator
Calculated serum osmolality · Osmol gap · Urine osmolality · SIADH / DI workup
Serum Values
mEq/L
Normal fasting: 70–99 mg/dL
Normal: 7–20 mg/dL
mOsm/kg
Leave blank if not available. Required to calculate osmol gap.
mg/dL
Include if ethanol level available — prevents false-positive osmol gap.
Formulas & Reference Ranges
Calc. Osmolality
2×Na + Glucose/18 + BUN/2.8 + EtOH/3.7 · Normal: 275–295 mOsm/kg
Osmol Gap
Measured − Calculated · Normal: ≤ 10 mOsm/kg
U/S Ratio
Urine Osm ÷ Serum Osm · > 1 = concentrating; < 1 = diluting
| Scenario | Serum Osm | Urine Osm | Likely Diagnosis |
|---|---|---|---|
| Hyponatraemia + concentrated urine | Low (< 275) | > 100 | SIADH, hypothyroidism, adrenal insufficiency |
| Hyponatraemia + dilute urine | Low (< 275) | < 100 | Primary polydipsia, reset osmostat |
| Hypernatraemia + dilute urine | High (> 295) | < 300 | Central or nephrogenic DI |
| Hypernatraemia + concentrated urine | High (> 295) | > 600 | Insensible losses (sweating, fever), inadequate intake |
| Elevated osmol gap | High or normal | — | Toxic alcohol (methanol, ethylene glycol), ethanol, mannitol, propylene glycol |
Sources: Verbalis JG. UpToDate 2024; Sterns RH. Sodium and Water Disorders. NEJM 2015.