How to Calculate Your Daily Water Needs
Water intake requirements vary significantly by body weight, activity level, climate and individual physiology. The baseline calculation uses 35ml per kg of body weight for adults — so a 70kg person needs approximately 2.45 litres as a baseline, adjusted upward for exercise, heat and other factors.
Signs of Dehydration
Even mild dehydration (1–2% of body weight) impairs cognitive performance and physical endurance. Signs include dark yellow urine, dry mouth, headache, fatigue, reduced concentration and dizziness. Thirst is a reliable but lagging indicator — by the time you feel thirsty, you're already mildly dehydrated. Regular sipping throughout the day is more effective than large amounts at once.
Water from Food
Approximately 20–30% of daily fluid intake comes from food. Fruits and vegetables are particularly high in water content: cucumber (96%), celery (95%), watermelon (92%), strawberries (91%). A diet rich in fruits and vegetables provides roughly 500–800ml of water per day without drinking any additional fluid.
Frequently Asked Questions
How much water should I drink per day?
The general NHS/health authority guideline is 6–8 glasses (about 1.2–2 litres) per day for adults. The US National Academies recommends about 3.7 litres total daily water for men and 2.7 litres for women — including water from food. Individual needs vary based on body size, climate, exercise and health status.
Does coffee count toward daily water intake?
Yes — coffee, tea and other caffeinated drinks do contribute to daily fluid intake. While caffeine has a mild diuretic effect, the fluid in the drink more than compensates. The old advice that coffee doesn't count has been revised — current evidence shows caffeinated drinks count toward your fluid totals.
How do I know if I'm drinking enough water?
The simplest check is urine colour. Pale yellow = well hydrated; dark yellow or amber = drink more; clear and very frequent = possibly overhydrated. Thirst is a reliable signal for most healthy adults, though elderly people may have reduced thirst sensation.
Does exercise change how much water I need?
Yes significantly. You can lose 0.5–2 litres of fluid per hour of exercise through sweat, depending on intensity and temperature. Add approximately 500ml for every 30 minutes of moderate exercise and 750ml–1L for intense sessions or hot weather exercise.
Can you drink too much water?
Yes — overhydration (hyponatraemia) can be dangerous, particularly during endurance events. It dilutes sodium in the blood and can cause nausea, headaches and in severe cases seizures. Drink to thirst during exercise rather than forcing excess fluid. The risk is low in everyday life but real in endurance sport.