It is common practice for clinicians to recommend a high fluid intake to produce dilute urine as a strategy to prevent future kidney stone development. For most patients with normal activity levels, fluid intake will need to modestly exceed the target urine output goal, as not all ingested fluid is excreted in the urine. For example, an intake of approximately 100 oz daily may yield a urine output of approximately 85 oz (approximately 2.5 L/day), although actual output will vary according to body size, diet, and ambient temperature. Patients with higher activity levels, chronic diarrhea, or greater heat exposure will require proportionately higher fluid intake to achieve the same target urine output. While the focus is on urine output rather than fluid intake, it is reasonable and practical to provide guidance to patients about different container volumes of fluid to aim to consume in order to likely achieve the recommended urine output. Figure 1 contains common container measures of fluid for reference. The recommended urine output of 2.5 L daily is supported by an RCT which evaluated a high-water intake intervention for the prevention of recurrent calcium-based kidney stones in adults. Borghi et al.,51 randomly assigned adults with a history of at least one prior symptomatic idiopathic calcium oxalate or calcium phosphate stone to one of two groups: subjects in the treatment group were encouraged to drink enough water to produce a urine output of at least two liters daily, while subjects in the control group were given no specific recommendations. At the end of the 5-year study period, patients in the high-water intake group produced greater volumes of urine and had significantly lower rates of stone recurrence than did the control group (12% versus 27% recurrence rates, respectively), representing a 55% relative risk reduction associated with high fluid intake. Additionally, the time to stone recurrence was significantly longer in the treatment group compared to the control group (39 months versus 25 months, respectively). The type of water best suited for stone prevention was addressed by Di Silverio et al.53 in an RCT comparing 2 L of oligomineral water with 2 L of tap water daily in adult patients with recurrent calcium stones treated in the past with SWL but stone-free at enrollment. At a mean follow-up of 19 months, there was no discernible difference in the rates of radiographic kidney stone recurrence between the 2 groups (17% versus 23%, respectively), although the rate of stone recurrence in both groups was relatively low. Encouraging patients to achieve a high urine volume with aggressive hydration can be a frustrating endeavor for clinicians. Desai et al. assessed the benefit of a combined behavioral intervention consisting of fluid prescription, financial incentives, health coaching, and patient-selected adherence strategies in achieving a high urine volume. Among adolescents and adults with a history of urinary stone disease and low urine volume who were randomly assigned to either standard preventative care or combined behavioral intervention, urine volume increased during the trial in both groups, but the increase in urine volume was greater in the intervention group compared to the control group at 6, 12, 18, and 24 months.127 Although the evidence is not robust, the available data supports the recommendation to maintain a high fluid intake in patients with recurrent calcium-based stones. This recommendation specifically focuses on a daily urine output of at least 2.5 L based on the mean urinary volume achieved in the high fluid group in the Borghi et al. trial,51 but in practice it also provides an additional margin to account for individual variation, potential measurement imprecision as well as to ensure adequate dilution of urinary stone-forming constituents throughout the day. Although the recommendation specifically identifies water as the preferred beverage, it is reasonable to encourage other beverages acknowledging that patients may prefer alternatives to water and these may be consumed and counted towards their total intake. Finally, this preventative therapy is inexpensive, readily available, and associated with minimal harm in most patients, making it a universal recommendation despite the limited evidence.