Water Intake Calculator | Know your daily water target

Work out how much water to drink each day from your body weight, exercise and climate. Shows the result in litres, millilitres, cups and glasses, with the calculation broken down.


Exercise per day
Clear
This is a general estimate. This calculator gives a rough daily target for a healthy adult. It is not medical advice. If you are pregnant or breastfeeding, have kidney or heart conditions, or take medication that affects fluid balance, your requirement can be very different and you should follow what your doctor tells you rather than this figure.

How much water do you actually need

The familiar advice of eight glasses a day is a rule of thumb with no strong evidence behind it. A more useful starting point is body weight: roughly 30 to 35 millilitres per kilogram for a healthy adult, which comes to about 2.1 to 2.5 litres for a 70 kg person. Exercise, heat and altitude all push it higher, and around 20 percent of most people intake arrives through food rather than drinks.

Thirst is a reasonable guide

For most healthy people with easy access to drinks, thirst regulates intake well enough without any counting. The main exceptions are older adults, whose thirst response weakens with age, and anyone exercising hard in heat, where sweat losses can outrun the thirst signal. In those situations drinking to a plan rather than to thirst makes sense.

What counts toward the total

All fluids count, not just plain water. Tea, coffee, milk, juice and soft drinks all contribute, and so does food: fruit, vegetables and soup are mostly water. Caffeine is a mild diuretic, but at ordinary intakes the fluid in a cup of coffee more than offsets the extra output, so coffee is a net gain rather than a loss.

Practical guidance

  • Urine colour is the simplest check. Pale straw is about right; dark yellow suggests you need more, and completely clear suggests you are drinking more than you need.
  • Spread intake through the day rather than drinking a lot at once, which mostly ends up as output.
  • For exercise, a rough guide is 400 to 800 ml per hour, more in heat and for heavier people.
  • Drinking far beyond thirst is not harmless. Very high intake can dilute blood sodium, a rare but serious condition called hyponatraemia.
  • Altitude, air travel, fever and breastfeeding all raise requirements noticeably.

Frequently asked questions

For a healthy adult, roughly 30 to 35 ml per kilogram of body weight is a reasonable starting point, which is about 2.1 to 2.5 litres for someone weighing 70 kg. Add roughly 350 to 700 ml for each hour of exercise, and more again in hot weather. Enter your details above for a figure based on your own weight and activity.

It is a memorable rule rather than a scientific finding, and no strong evidence supports it as a universal target. Eight 250 ml glasses is two litres, which happens to be in the right region for an average adult, so it is not wrong, just imprecise. Someone weighing 50 kg and someone weighing 100 kg clearly do not need the same amount.

Yes. Caffeine has a mild diuretic effect, but at normal consumption the fluid in the drink far outweighs the extra urine produced, so tea and coffee are a net contribution. The same applies to milk, juice and soft drinks, although those bring sugar and calories with them.

A common guide is 400 to 800 ml per hour of activity, toward the higher end in heat or if you are heavier or sweat a lot. For sessions over about an hour, especially in heat, replacing electrolytes matters as well as fluid, because sweat carries sodium out with the water.

Yes, though it is uncommon. Drinking very large volumes in a short time can dilute blood sodium to dangerous levels, a condition called hyponatraemia, which has caused deaths in endurance events and water-drinking contests. Healthy kidneys handle roughly one litre per hour at most, so spreading intake through the day is safer than drinking a lot at once.

Urine colour is the easiest indicator. Pale straw suggests you are about right, dark yellow or amber suggests you need more, and water-clear urine all day suggests you are drinking more than necessary. Persistent thirst, headaches and tiredness can also indicate you are behind.

It does, and by a meaningful amount. Around 20 percent of total intake typically comes from food for most people. Fruit, vegetables, soup, yoghurt and rice are all substantially water, so someone eating a lot of fresh produce genuinely needs to drink less than someone living on dry food.

Yes, and often considerably more. Sweat losses in heat and humidity can reach a litre or more per hour during exertion. Cold weather also raises requirements slightly, partly through respiratory losses and partly because thirst is blunted in the cold, which makes cold-weather dehydration easy to miss.

How much water should you drink a day?

A practical starting point is 30 to 35 ml per kilogram of body weight, so about 2.1 to 2.5 litres for a 70 kg adult. Add roughly 350 to 700 ml per hour of exercise and more in heat. The eight-glasses rule is a rough approximation, not a scientific target.