How much water do I really need to drink per day?

The commonly used recommendation is approximately 1.5 litres of drinking water per day for a sedentary adult. Water fulfils many essential functions: it is a building block for cells, a carrier for nutrients and waste products, helps regulate body temperature, and lubricates joints. Young children and older adults are particularly vulnerable to dehydration and should pay extra attention to this.

However, 1.5 litres is not a universal standard. During intense exercise in hot weather, athletes can lose 4 to 10 litres of fluid per day through sweat, along with large amounts of salt (sodium). People who exercise therefore need to drink considerably more than the standard rule suggests, and should also focus on replenishing sodium, not just water. For ordinary recreational exercise this can be managed through normal eating and drinking, but when dehydration exceeds 5% of body weight, more active replenishment is needed.

A systematic review of 18 randomised studies (published in JAMA Network Open, 2024) shows that the scientific evidence for the broader health benefits of drinking extra water is still limited. The most consistent benefits have been found for weight loss (44 to 100% greater loss than in control groups) and fewer kidney stones (15 fewer cases per 100 people over 5 years). For other outcomes such as fewer migraines, fewer urinary tract infections, or better blood sugar control, only one study per outcome is available, which is too few for firm conclusions. Moreover, 44% of the studies examined found no positive result.

A small randomised study in Japanese adults did show that drinking extra water, around 700 ml per day more than usual for 12 weeks, lowered blood pressure, diluted waste products in the blood, and protected kidney function. It had no effect on blood sugar levels. There are also indications that drinking water slightly increases energy expenditure via the nervous system and stimulates fat burning, but this effect is small and the evidence is still limited.

A practical consideration concerns the water source. Someone who meets their entire daily fluid needs from bottled water ingests an estimated 90,000 additional microplastic particles per year through the water, compared with around 4,000 for tap water only. The health risks of this are not yet well understood, but it is a reason to prefer tap water when its quality is good.