How your water target is calculated

This tool uses a widely used clinical guide of about 35 ml of fluid per kilogram of body weight as a baseline, then adds an allowance for activity — exercise increases losses through sweat and breathing, so more active people need more.

For example, a 70 kg person has a baseline of roughly 2.45 litres, rising with training. The result is shown in litres and fluid ounces so it works wherever you live. Remember this is total fluid: drinks of all kinds count, and food (especially fruit and vegetables) provides around 20% of it.

Signs you are well hydrated

The simplest check is the colour of your urine: pale straw means you are well hydrated, while dark yellow signals you need more fluid. Thirst, headaches, tiredness and poor concentration can all be early signs of mild dehydration.

There is no need to force large amounts at once. Sipping steadily through the day is more effective, and drinking to thirst plus the guidance above keeps most healthy adults in good balance.

When you need more — or less

Increase your intake in hot or humid weather, at altitude, during illness with fever, and during pregnancy or breastfeeding. Endurance athletes lose significant fluid and electrolytes in sweat and should plan their drinking around training.

A small number of people — for example those with certain heart or kidney conditions — need to limit fluids, and should follow their doctor’s advice rather than a general formula. Drinking extreme amounts very quickly can dilute blood sodium dangerously, so aim for steady, sensible hydration.

Frequently asked questions

Sources & references

1. Institute of Medicine (National Academies). "Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate." Washington, DC: National Academies Press; 2005.
2. EFSA Panel on Dietetic Products, Nutrition, and Allergies. "Scientific Opinion on Dietary Reference Values for water." EFSA Journal. 2010;8(3):1459.