For healthcare professionals
Urine Osmolal Gap Calculator
The urine osmolal gap is the difference between the measured urine osmolality and the osmolality calculated from urine sodium, potassium, urea and glucose. The gap is made up mainly of ammonium salts, so in a normal anion gap (hyperchloremic) metabolic acidosis it gives an indirect estimate of how much acid the kidney is excreting as ammonium. This tool is written for clinicians and is not a diagnosis.
Urine values
Use values from the same spot urine sample. Enter 0 for glucose if none is detected. Nothing you enter leaves this page.
What you will see
The calculated urine osmolality, the urine osmolal gap with the working, and how a gap under or over 150 mosmol/kg is read in a normal anion gap metabolic acidosis.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formula and the 150 mosmol/kg reading as printed in Brunner R and colleagues, Critical Care 2015 (Background and Methods), with the normal range, the ammonium approximation and the limits from the review by Alamilla-Sanchez M and colleagues, Pathophysiology 2025 (section 3.6). Both are open access (CC BY 4.0). The tests check the formula, the reading on each side of 150 and at exactly 150, and that a value near 150 is never rounded onto it.
Validated in
Brunner 2015 used the gap in 100 consecutive adults in a medical intensive care unit in Austria, 43 of whom had a hyperchloremic acidosis; the 150 mosmol/kg reading was taken from earlier work and was not itself validated against an outcome there. Alamilla-Sanchez 2025 is a narrative review.
Limits
The gap is an approximation of urine ammonium and should not be used as an exact measure of it (Alamilla-Sanchez 2025). The reading applies only to a hyperchloremic acidosis with preserved kidney function (GFR over 25 ml/min) (Brunner 2015). Urinary tract infection with urease-producing bacteria limits its use (Alamilla-Sanchez 2025). Urea and glucose are entered in mg/dL, as in the published formula. Brunner 2015 gives osmolality in mosmol/kg and the review in mOsm/L; the tool uses mosmol/kg. Brunner 2015 writes 'less than 150' and 'more than 150' in its Methods but describes renal tubular acidosis as a gap of 150 mosmol/kg or less in its abstract and Results; because the paper is not consistent at exactly 150, the tool gives no reading at exactly 150.
The formula
Calculated urine osmolality (mosmol/kg) = 2 x (urine sodium + urine potassium, mmol/L) + urine urea nitrogen (mg/dL) / 2.8 + urine glucose (mg/dL) / 18 (Brunner 2015).
Urine osmolal gap = measured urine osmolality - calculated urine osmolality (Brunner 2015).
Worked example: urine sodium 50 and potassium 40 mmol/L, urea nitrogen 560 mg/dL and no glucose give a calculated osmolality of 2 x 90 + 200 + 0 = 380 mosmol/kg. With a measured osmolality of 600 mosmol/kg, the gap is 600 - 380 = 220 mosmol/kg.
Reading the result
Sodium, potassium, urea and glucose account for most urine solutes; the gap left over is an approximate measure of unmeasured urine ammonium salts, and the main kidney response to a metabolic acidosis is to excrete more ammonium (Brunner 2015). In a hyperchloremic acidosis:
- Under 150 mosmol/kg, with preserved kidney function (GFR over 25 ml/min): an inadequate kidney response, characteristic of renal tubular acidosis (Brunner 2015). In distal renal tubular acidosis the gap is usually below 150 (Alamilla-Sanchez 2025).
- Over 150 mosmol/kg: suggests bicarbonate loss outside the kidney (Brunner 2015). The review notes a gap of about 400 when ammonium excretion is very high, as in toluene intoxication and chronic diarrhea (Alamilla-Sanchez 2025).
Outside of acidosis, the review gives a normal gap of 10 to 100 and describes about half of the gap as ammonium (Alamilla-Sanchez 2025). Unlike the urine anion gap, the urine osmolal gap is not altered by large amounts of unmeasured anions in the urine, which is why it is preferred in critically ill patients (Brunner 2015) (Alamilla-Sanchez 2025).
Related tools
The urine anion gap calculator gives the simpler estimate from sodium, potassium and chloride. For a high potassium, the TTKG calculator estimates collecting duct potassium secretion. For the blood side, see the serum osmolality calculator and the anion gap calculator. Other bedside scores are listed under For healthcare professionals.
Sources
- Brunner R, Drolz A, Scherzer TM, Staufer K, Fuhrmann V, Zauner C, et al. Renal tubular acidosis is highly prevalent in critically ill patients. Crit Care. 2015;19(1):148. doi:10.1186/s13054-015-0890-0. PMID 25888397. Open access (CC BY 4.0). Background and Methods (urine osmolal gap formula and the 150 mosmol/kg reading with preserved renal function). pmc.ncbi.nlm.nih.gov/articles/PMC4404695/
- Alamilla-Sanchez M, Alcala Salgado MA, Ulloa Galvan VM, Yanez Salguero V, Yama Estrella MB, Morales Lopez EF, et al. Understanding renal tubular function: key mechanisms, clinical relevance, and comprehensive urine assessment. Pathophysiology. 2025;32(3):33. doi:10.3390/pathophysiology32030033. PMID 40700075. Open access (CC BY 4.0). Section 3.6 (urine osmolal gap: normal range, ammonium approximation and limits). pmc.ncbi.nlm.nih.gov/articles/PMC12285995/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.