Calculate serum osmolality, osmolar gap, urine osmolality status, and the urine/plasma (U/P) osmolality ratio — with instant interpretation.
Enter serum values
Sodium, glucose and BUN/urea are required. Ethanol and measured osmolality are optional.
mEq/L
mg/dL
mOsm/kg
Calculated Serum Osmolality
– mOsm/kg
–
LowNormalHigh
Reference: 275–295 mOsm/kg. Formula estimate — not a substitute for direct lab measurement.
Osmolar gap & measured osmolality
Measured Osmolality
–
Osmolar Gap
–
Enter a measured (lab) osmolality above to calculate the osmolar gap.
Formula
Posm = 2 × Na + Glucose ÷ 18 + BUN ÷ 2.8 (+ Ethanol ÷ 4.6)Posm (SI) = 2 × Na + Glucose (mmol/L) + Urea (mmol/L)Osmolar Gap = Measured Osmolality − Calculated Osmolality
About calculated osmolality & the osmolar gap
Calculated (estimated) serum osmolality approximates the true, lab-measured osmolality using the three major osmotically active solutes: sodium, glucose, and urea/BUN. The osmolar gap is the difference between what the lab actually measures (by freezing-point depression osmometry) and what the formula predicts.
Reference ranges
Serum osmolality: 275–295 mOsm/kg (varies slightly by lab/method)
Osmolar gap: normally ≤10 mOsm/kg; some laboratories accept up to 15 mOsm/kg as within normal variability
Causes of an elevated osmolar gap
Toxic alcohol ingestion — methanol, ethylene glycol, isopropanol (most clinically important cause to exclude)
Unaccounted ethanol (if not included in the calculation)
Mannitol infusion, IV contrast, glycine (post-TURP irrigation)
Severe diabetic ketoacidosis or lactic acidosis (unmeasured ketones/lactate)
Advanced renal failure
Pre-analytical/lab error, or severe hyperlipidemia/hyperproteinemia artifact
High / hypertonic (>295): water deficit or solute excess — dehydration, uncontrolled hyperglycemia (HHS/DKA), diabetes insipidus, hypernatremia. Values above ~320 mOsm/kg are clinically significant and warrant urgent evaluation.
Enter urine & serum values
mOsm/kg
mOsm/kg
Auto-filled from measured (or calculated) serum osmolality.
Urine Osmolality
– mOsm/kg
–
DiluteSubmax.ConcentratedMax.
Spot urine osmolality is highly dependent on current hydration status — interpret alongside clinical context.
Urine / Plasma (U/P) Osmolality Ratio
U/P Osmolality Ratio
–
Enter urine osmolality, and provide a serum osmolality (auto or manual), to calculate the ratio.
Formula
U/P Osmolality Ratio = Urine Osmolality ÷ Serum (Plasma) Osmolality
Interpreting urine osmolality
<100 mOsm/kg (dilute): maximally dilute urine — appropriate response to water excess, or seen in primary polydipsia, low solute intake ("beer potomania"), or diabetes insipidus
100–300 mOsm/kg (submaximal): intermediate concentrating ability; in the setting of hyponatremia, this range is often inappropriate and suggests some ADH activity
300–800 mOsm/kg (concentrated): typical for a normally hydrated to mildly concentrated random/spot sample
>800 mOsm/kg (highly concentrated): near-maximal renal concentration — appropriate with volume depletion, or seen with persistent ADH activity (e.g. SIADH) despite normal/low plasma osmolality
Clinical uses of the U/P ratio
Hyponatremia workup: a urine osmolality >100 mOsm/kg in a hyponatremic patient indicates ADH is active (SIADH, hypovolemia, heart failure, cirrhosis) rather than primary polydipsia or low solute intake
Acute kidney injury workup: a high U/P ratio (well-concentrated urine, generally >1.3) supports intact tubular concentrating function and favors a prerenal cause; a ratio near 1.0 ("isosthenuria") suggests impaired concentrating ability, as in acute tubular necrosis or intrinsic renal disease
Diabetes insipidus evaluation: persistently dilute urine (ratio <1) despite rising plasma osmolality suggests central or nephrogenic DI
These are general teaching thresholds — always correlate with the full clinical picture (volume status, medications/diuretics, FeNa, BUN/creatinine ratio) rather than any single value in isolation.
