A calculation used to identify if watery diarrhea has an osmotic or secretory etiology. Calculated with the equation: $290-[2\times (\text{stool sodium}+\text{stool potassium})]$, with 290 mOsm representing stool osmolarity. A low stool osmotic gap (< 50) is suggestive of secretory diarrhea; a high gap (> 100) is suggestive of osmotic diarrhea.