On average, men need about 3-3.7 litres of water a day, while women need around 2.5-2.7 litres. "Those...numbers are generally correct..but they should be viewed as a guideline...not a strict rule," Dr Amit Saraf, director, internal medicine, Jupiter Hospital said. A better strategy is to sip water throughout the day instead of drinking large amounts at once, Dr Saraf added.