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

On average, a sedentary adult needs around 1.5 litres of water per day to maintain fluid balance. That figure is derived from experimentally determined averages for healthy people, not from a medical minimum. Exercise, hot weather, illness or pregnancy significantly increase the requirement: athletes training intensively in the heat can lose up to 10 litres of fluid per day through sweating, and must replenish this with water and sodium.

Thirst is a reasonably reliable indicator, but not for everyone. Young children and older adults are less aware of dehydration and are therefore particularly vulnerable. Losing more than 1% of your body water can already lead to headaches, difficulty speaking, muscle weakness and low blood pressure when standing up. You can use the colour of your urine as a handy signal: pale yellow is generally a sign of good hydration.

Drinking more than you strictly need sometimes offers additional benefits, but not always. One small study found that an extra half litre of water per day over three months led to a slight reduction in blood pressure and a modest improvement in kidney function. A broader analysis of 18 studies found limited evidence for fewer kidney stones and slightly greater weight loss, but 44% of the studies found no positive effect at all. For migraines, urinary tract infections and diabetes, the evidence is considered too thin for firm conclusions.

Whether you drink tap water or bottled water matters for your exposure to microplastics. Someone who drinks only bottled water is estimated to ingest around 90,000 additional microplastic particles per year, compared with approximately 4,000 via tap water. What this means for your health in the long term is not yet known. But given this difference, good-quality tap water is the more practical choice for that reason as well.