Kidneys have tremendous concentrating ability and may reduce daily urine output to 600 ml, and still carry on normal renal functions. Daily insensible water loss in air-conditioned environment is approximately 300 ml (through breathing). So minimum fluid intake has to be about a litre, and proportionately more in summers to account for losses via sweating. Keeping urine output at minimum does not damage kidneys but urine remains highly concentrated and supersaturated, making such people prone to stone formations in warm climate. Ideal urine output to prevent stone formation is about 2 to 2.5 litres, for which you must take about 3 litres fluids every day. The fluid intake may be as beverages or any kind of liquids. Even solid foods have a high water content.