Water Intake Calculator

Daily water needs depend on body weight, activity level, climate and overall health. A common starting point is 30 to 35 ml per kilogram of body weight, so a 70 kg person lands at 2.1 to 2.45 litres of total fluid. The NHS Eatwell Guide puts the minimum at 6 to 8 glasses a day, roughly 1.2 litres, counting water, lower-fat milk, sugar-free drinks, tea and coffee. EFSA reference values are higher at 2.5 litres a day for men and 2.0 litres for women, though those include the water held in food.

The calculator takes your weight and activity level and turns them into a personalised daily target in litres. Exercise adds roughly 500 to 1,000 ml per hour of moderate to vigorous activity. Hot or humid weather, illness with a fever, and pregnancy or breastfeeding all push the figure up further. Someone of 75 kg training three times a week comes out near 3.0 litres, which is about 12 to 13 glasses of 250 ml.

All fluids count towards the total, including tea, coffee, milk and juice, and so does the water in food, which contributes roughly 20% of daily intake through fruit, vegetables and soups. Watermelon is 92% water and cucumber 96%. Caffeinated drinks do count, despite the persistent myth, because their mild diuretic effect is offset by the fluid they bring with them. The old advice about 8 glasses was always about all fluids and food moisture rather than water alone.

Several situations raise the requirement well above baseline. Hot weather adds 500 to 1,000 ml a day, and exercise 500 to 750 ml an hour at moderate effort or over 1,000 ml an hour when it is vigorous. A fever above 37.5°C adds about 200 ml a day for every degree. Diarrhoea and vomiting need rehydration salts rather than plain water, with NHS Dioralyte sachets made up in 200 ml. Pregnancy adds 300 ml a day and breastfeeding 700 ml. Diabetes increases the need through polyuria, while heart and kidney conditions can mean a fluid restriction instead, so take medical advice.

UK tap water is among the safest in the world and there is little reason to buy bottled. The Drinking Water Inspectorate regulates more than 50 parameters and compliance runs at 99.97%. Hardness varies sharply by region, from very hard water in South-East England at over 300 mg/L of calcium carbonate to soft water under 60 mg/L in Scotland and the South-West. Hard water leaves limescale but is safe and may contribute usefully to calcium intake. The NHS treats bottled water as unnecessary for health, and its environmental impact is 1,000 times that of tap. Filter jugs improve the taste by removing chlorine and rarely do more than that.

Dehydration announces itself in stages. Mild dehydration, at 1 to 2% of body weight lost, brings thirst, dark yellow urine, a slight headache and fatigue. At 3 to 4% you get strong thirst, a dry mouth, less urine, dizziness and irritability. Beyond 5% the heart rate rises and blood pressure falls with confusion, which is a medical emergency. Urine colour is the simplest check, with pale straw the target and dark amber a signal to drink. Overhydration is rare but real, since hyponatraemia can follow extreme intake above 10 litres a day, causing nausea, headache, confusion and swelling, and marathon runners have died from drinking too much, which is why endurance events push sodium tablets and sports drinks.

Not every fluid is equally useful. Plain water is the sensible default, with no calories, no sugar and no caffeine, but herbal tea, milk, sugar-free squash and soups all count too, and milk adds electrolytes and protein after exercise. Coffee at 200 to 300 mg of caffeine, which is two or three cups, is mildly diuretic and still a net gain in fluid. Alcohol is the exception, since beer counts as fluid by volume while the alcohol itself causes a net loss of water. Energy drinks are high in both sugar and caffeine and are worth limiting, and sugar-free flavoured water suits anyone who dislikes plain.