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Baseline needs are weight × 35 ml/kg, plus about 350 ml per 30 minutes of exercise, plus a climate bonus (temperate 0, hot/humid 500, hot/dry 750, cold 300 ml). The total is shown in litres and in 250 ml glasses. Results are estimates for planning, not professional medical advice, follow a physician-set limit if one applies to you.
Water needs scale with body mass, because larger bodies have more cells, more blood volume and higher baseline activity. The widely used clinical rule of thumb is 30–35 ml of water per kilogram of body weight per day for baseline fluid needs in temperate conditions.
This calculator uses the additive formula: daily water (ml) = body weight (kg) × 35 + (exercise minutes ÷ 30 × 350) + climate bonus, where the bonus is 0 ml for a temperate climate, 500 ml for hot/humid, 750 ml for hot/dry and 300 ml for cold. Results are shown in litres and in 250 ml glasses.
Worked example, an 80 kg person doing a 45-minute workout in a temperate climate: baseline 80 × 35 = 2,800 ml; exercise addition 45 ÷ 30 × 350 = 525 ml; total 3,325 ml ≈ 3.3 litres (about 113 oz, or 13 standard 250 ml glasses). That is roughly six and a half 500 ml bottles across the day.
The output is a general guideline for healthy adults. About 20% of daily water normally comes from food, so drinking targets sit below total-fluid reference values.
Enter your body weight in kilograms, how many minutes of moderate-to-vigorous exercise you do per day, and select the climate you live and work in. Click Calculate to see your recommended daily water intake in litres and in 250 ml glasses.
Track your intake through the day against the glasses figure, it is the easiest way to keep score. The two simplest checks that you are drinking enough are thirst and urine colour: pale straw suggests good hydration, dark yellow suggests you are behind.
If you have heart, kidney or liver disease, or take diuretics, your right fluid intake is a medical decision, confirm it with your physician rather than using this calculator.
Partly. Eight 250 ml glasses is 2 litres: a reasonable landing zone for a smaller adult and an underestimate for a larger or very active one. A 50 kg office worker and a 100 kg construction worker in summer need very different amounts.
The rule's real problem is that it was never personal, it came from a 1945 recommendation that was misinterpreted along the way. Weight-based targets, plus listening to thirst and urine colour, beat any fixed number for everyone.
For context, the NHS recommends 6–8 glasses of fluid a day (roughly 1.5–2 litres) for most adults, while the US National Academy of Medicine puts adequate total water at about 3.7 litres a day for men and 2.7 for women, figures that include all fluids and the water in food. Your personal number sits somewhere between those, which is exactly what the weight-based calculation finds.
Yes, all fluids count toward the total. Water, tea, coffee, milk, soups, fruit and vegetables all contribute; fruit, vegetables and soups are 80–95% water. About 20% of a typical person's daily water comes from food alone.
Does coffee count as water intake? Yes. At normal intakes, caffeine's mild diuretic effect is far too weak to cancel the fluid it delivers, so coffee and tea count toward your total. Alcohol is the exception, it genuinely dehydrates.
Sparkling water hydrates exactly as well as still: carbonation changes the mouthfeel, not the hydration. Only heavily sugared drinks carry a downside. Still, plain water remains the best default because it comes without sugar or calories, replacing sugary drinks with water also saves hundreds of real calories a day, which modestly helps weight loss plans.
Sweat changes the math faster than any other factor. The calculator adds roughly 350 ml per half hour of exercise, which approximates sweat losses from moderate workouts, about 0.5–1 litre per hour of moderate exercise, rising to 1–2 litres per hour in hot conditions.
The practical protocol around a session: drink about 400–600 ml in the two hours before training, 150–250 ml every 15–20 minutes during it, and afterwards replace roughly 1.5 litres per kilogram of body weight lost. Sessions under an hour need only water; beyond that, or in heat, drinks with electrolytes help replace the sodium lost in sweat.
Also note the counter-risk: endurance athletes drinking far beyond thirst are the classic hyponatremia case. During long events, drink to thirst, not to a fixed schedule.
Yes, and it is worth taking seriously. Drinking large volumes faster than the kidneys can excrete (roughly a litre per hour) can dilute blood sodium, a condition called hyponatremia or water intoxication, which in severe cases is dangerous.
Healthy kidneys handle even 5+ litres spread across a day; the danger is speed, not totals, more than about a litre per hour sustained. So sip through the day rather than forcing litres in one sitting.
This is mostly a risk in endurance events and extreme water challenges, not normal life. People with heart failure or kidney disease may need a physician-set fluid limit rather than a target, follow medical advice over any calculator, and never apply a general target if you have been told to restrict fluids.
Knowing your litres and drinking them are different projects. The mechanics that work in behavioural studies and in practice:
Anchor water to existing habits, a full glass on waking (seven-plus hours without intake leave most people a few hundred millilitres down by morning), one before each meal, one at every coffee. Pairing beats willpower.
Keep the bottle visible, intake tracks proximity with almost embarrassing reliability, so a bottle on the desk outperforms any reminder app.
Front-load the day, covering two-thirds of your target by mid-afternoon hydrates the hours that matter and spares the 3 a.m. bathroom trip.
Make it palatable, temperature and a slice of lemon or cucumber move real volume, and count fully.
Heat, travel and illness spike needs fast: hot humid weather can add a litre or more before any exercise; air travel dries you quietly (a glass per hour aloft blunts the arrival headache); and fever adds roughly 500 ml per degree above normal, when oral rehydration solutions beat plain water.
A practical rule is 30–35 ml per kilogram of body weight per day, about 2.4 litres for a 70 kg adult, plus roughly 350 ml per 30 minutes of exercise and 500 ml extra in hot climates. Enter your numbers above for your personal target.
For a smaller, lightly active person in a mild climate, often yes. For a heavier or very active person it is usually short: an 85 kg person's baseline alone is about 3 litres by the weight-based rule. Personalise it rather than trusting a fixed number.
Yes. At normal intakes, caffeine's mild diuretic effect does not cancel the fluid it delivers, coffee, tea and milk all count toward your daily total. Alcohol is the exception: it genuinely dehydrates. Plain water is still the best default.
The two simplest checks are thirst and urine colour. Pale straw suggests good hydration; dark yellow suggests you are behind. Headaches, fatigue and poor concentration are common early signs of mild dehydration, and dark yellow or strong-smelling urine confirms it.
Modestly. Drinking about 500 ml before meals increases fullness and trimmed intake in trials, and replacing sugary drinks with water saves real calories. It supports a plan built on diet and activity; it does not replace one.
Yes. Sustained intake above about a litre per hour can outpace the kidneys and dilute blood sodium (hyponatremia). Healthy kidneys handle 5+ litres spread across a day, the danger is speed, not totals. Sip through the day rather than forcing volume at once.
Yes, carbonation changes the mouthfeel, not the hydration. Plain sparkling water counts fully toward your total; only heavily sugared versions carry a downside.
Yes. The calculator adds roughly 350 ml per half hour of exercise, and sweaty sessions in heat can need more. For efforts over about an hour, drinks with electrolytes help replace the sodium lost in sweat.
They do: pregnancy adds roughly 300 ml a day, and breastfeeding adds roughly 700 ml a day, since milk itself is about 90% water. If you have been given a fluid restriction during pregnancy, that advice comes from your obstetrician and overrides any calculator.