Hydration Calculator
Daily water requirement varies by body weight, activity level, climate, and individual factors like medication or pregnancy. This calculator gives a personalised target in ounces, litres, and cups — including the bump for exercise and hot conditions.
How much water do I need per day?
The standard baseline is about 33 ml per kilogram of body weight. For a 70 kg adult, that is 2.3 litres per day. The figure includes water from all sources — drinking water, beverages, and food. About 20% of daily water comes from food (fruit, vegetables, soup, yoghurt), so the drinking-water target is roughly 1.8 litres for the same person.
Why activity level matters
Exercise increases water loss through sweat and respiration. A sedentary office worker loses about 2-2.5 litres per day; an athlete in training can lose 4-6 litres. The calculator applies a multiplier: 1.0 for sedentary, 1.15 for moderate, 1.3 for active, 1.5 for athlete. For specific events lasting over an hour, add 500-1000 ml per hour of exercise on top of the daily baseline.
Climate and altitude effects
Hot climates (30°C+) increase requirement by about 700 ml per day from sweat. Warm climates (22-30°C) add about 300 ml. Cold climates slightly reduce requirement. High altitude (above 2,500 m) increases water loss through faster respiration — add 250-500 ml per day at altitude. Dry indoor air in winter (low humidity) also raises requirement slightly.
Signs you are not drinking enough
Dark yellow urine (target: pale straw colour). Headache without obvious cause. Fatigue, especially mid-afternoon. Dry mouth and lips. Constipation. Lightheadedness on standing. Mild dehydration (1-2% body water deficit) impairs concentration and exercise performance noticeably. Severe dehydration (5%+) requires medical attention.
Frequently asked questions
How many glasses of water should I drink a day?
The classic '8 glasses' equals about 1.9 litres — close to the standard adult daily requirement. For most adults the personal target…
The classic '8 glasses' equals about 1.9 litres — close to the standard adult daily requirement. For most adults the personal target is 33 ml per kg of body weight, adjusted for activity and climate. The 70 kg adult needs about 2.3 litres total, with 1.8 litres coming from drinks.
Does coffee count as water?
Mostly yes. The mild diuretic effect of caffeine is small — about 100 ml extra urine per 100 mg of caffeine. A…
Mostly yes. The mild diuretic effect of caffeine is small — about 100 ml extra urine per 100 mg of caffeine. A 200 mg coffee in 240 ml of water still gives you about 140 ml of net hydration. Tolerant daily drinkers experience an even smaller diuretic effect. Tea, juice, milk, and soda all count toward hydration.
Can I drink too much water?
Yes, hyponatraemia is a real risk — diluting blood sodium too much. Usually only happens at intakes above 5-6 litres in a…
Yes, hyponatraemia is a real risk — diluting blood sodium too much. Usually only happens at intakes above 5-6 litres in a short period, especially during endurance exercise with electrolyte loss. Symptoms: headache, nausea, confusion. For typical daily life, drinking 3-4 litres is safe; problems start much higher.
Do I need to drink more during pregnancy?
Yes — about 300 ml extra per day during pregnancy, increasing to 700 ml extra during breastfeeding. The extra water supports increased…
Yes — about 300 ml extra per day during pregnancy, increasing to 700 ml extra during breastfeeding. The extra water supports increased blood volume, amniotic fluid, and milk production. The Institute of Medicine guideline is 2.3 litres per day during pregnancy and 2.7 litres during breastfeeding (total water from all sources).
Should I drink water before or after meals?
Either works. Old advice to avoid water with meals (because it dilutes stomach acid) has been disproven — water is absorbed quickly…
Either works. Old advice to avoid water with meals (because it dilutes stomach acid) has been disproven — water is absorbed quickly and does not affect digestion. Drinking water 30 minutes before a meal slightly reduces calorie intake in some weight-loss studies. Drinking water with meals helps with satiety and digestion.
How can I tell if I'm hydrated?
Urine colour is the easiest check — pale straw colour is well hydrated; dark yellow means drink more. Thirst is a reliable…
Urine colour is the easiest check — pale straw colour is well hydrated; dark yellow means drink more. Thirst is a reliable signal but kicks in slightly after mild dehydration. Frequency: most adults urinate 6-8 times per day when adequately hydrated. Skin elasticity (pinch the back of the hand; it should snap back instantly) is another quick check.