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Calculator: Urine osmolal gap (SI units)*

Calculator: Urine osmolal gap (SI units)*

Measured urine osmolality  
Urine Na  
Urine K  
Urine urea nitrogen  
Urine glucose  

Important: Inputs must be complete to perform calculation.

Urine osmolal gap  
Decimal precision  


  • A urine osmolal gap of less than 150 mosm/kg in a patient with chronic metabolic acidosis suggests that NH4+ excretion is impaired. Because NH4+ excretion is reduced in patients with distal RTA, a low urine osmolal gap is consistent with this diagnosis.
  • When the urine osmolal gap exceeds 400 mosm/kg, it is likely that the urine NH4+ concentration is 200 mEq/L or greater. This would be expected with hyperchloremic metabolic acidosis generated by chronic diarrhea.
  • Urine glucose can be assumed to be 0 mmol/L if glucose is not detected by urine dipstick testing.
  • Urine Na: urine sodium; Urine K: urine potassium; RTA: renal tubular acidosis.
  • * This calculator uses SI units (Système international d'unités) as the default for inputs. Do not use this calculator if you are using conventional units (also known as US units and gravimetric units); please refer instead to the conventional units version of this calculator.

Equations used
Urine osmolal gap = Measured urine osmolality - ((2 * (Urine Na + Urine K)) + Urine urea nitrogen + Urine glucose)


Only digits 0 to 9 and a single decimal point (".") are acceptable as numeric inputs. Attempted input of other characters into a numeric field may lead to an incorrect result.

Information on this page may not appear correctly if printed.

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