Daily water intake:
litres per day by body weight, and the caveats

A weight-based starting point

A widely used rule of thumb sets daily fluid intake at 30 to 35 ml per kilogram of body weight. For a 70 kg adult, that is 2.1 to 2.45 litres a day. It is not a law of nature or a medical prescription, but it is simple, scales with body size and gives a number you can adjust.

Body size matters because water needs follow body mass and metabolism: a larger person loses more water through breath, sweat and urine. Other sources set a single figure for everyone, such as 2 to 3 litres, which suits some people and over- or under-shoots for others.

Use the weight-based figure as a starting range, then adjust for your day. The sections below show how, and where the rule stops being useful.

The water intake formula

The result is total fluid, which can come from water, tea, coffee, milk, soup and the water in fruit and vegetables. A typical diet supplies roughly 20% of total water intake from food, so the amount you need to drink is a little lower than the total.

Daily water by body weight

The table is for a normal day in a temperate climate with light activity. The glass count rounds the range to whole glasses, and a glass can be any size, so think in litres if you use a bottle.

Worked example: a 62 kg office worker

A 62 kg person works at a desk in an air-conditioned office, walks 30 minutes a day and lives in a mild climate. They choose the middle of the range at 33 ml per kg.

Of that, perhaps 400 to 500 ml comes from food, which leaves about 1.5 litres to drink. A bottle of 750 ml, filled twice, covers the day. If the same person exercises for an hour in warm weather, they would add some extra fluid on top.

When to drink more

The base figure assumes a typical day. Several things raise needs, and the adjustments below are approximate, so use thirst and urine colour as the guide.

• Exercise: a common allowance is 400 to 800 ml extra per hour of moderate activity, more in heat.
• Hot or humid weather: sweat losses can rise by a litre or more a day.
• Illness: fever, vomiting or diarrhoea increase losses and may call for oral rehydration solution.
• Pregnancy and breastfeeding: official guidance sets higher fluid intakes, so follow your doctor's advice.
• High altitude and dry air: faster breathing loses more water.

Pale straw-coloured urine and rarely feeling thirsty suggest you are well hydrated, while dark yellow urine suggests you need more fluid.

Spreading intake across the day

Hitting 2 litres is easier when it is planned. For the 62 kg office worker above, aiming to drink about 1.5 litres plus food and tea might look like this: 300 ml on waking, 250 ml with breakfast, 250 ml mid-morning, 300 ml with lunch, 250 ml mid-afternoon, and 250 ml in the evening. That adds up to 1,600 ml before counting soup, fruit and the milk in tea.

Habits help more than willpower. Keeping a bottle in view, drinking a glass when you take any medicine and having a drink with each meal are cues that need no effort. Flavouring with lemon, mint or cucumber can make plain water more appealing, with no added sugar.

Needs and thirst can change with age. Older adults often feel thirst less strongly and may need a prompt to drink regularly, while children lose water quickly in hot weather and during play. If you look after either, offer drinks routinely.

Keep alcohol and very sugary drinks in check, since they add calories and, for alcohol, increase fluid loss. If you track intake with an app, treat it as a rough guide and let thirst and urine colour act as the final check.

Finally, treat any online water formula as a guide to be tested against your own life. Try the weight-based range for a week and notice your energy, headaches and urine colour. If you consistently feel thirsty, add a glass or two. If you are visiting the bathroom very often and drinking beyond thirst, you can ease back. Your doctor can help set a target if you have a medical condition or take medicines that affect fluid balance.

Caveats: when the rule does not apply

A weight formula ignores individual health. People with kidney disease, heart failure, liver disease, or those taking certain medicines may be told to limit fluids, and a general rule can be harmful for them. Follow your doctor's number.

Drinking too much is also possible. Large volumes in a short time, particularly during endurance events, can lower blood sodium (hyponatraemia), which causes headaches, nausea and in severe cases confusion. Spread intake through the day and drink to thirst during long exercise.

Official reference values differ by body size and region. The European Food Safety Authority gives adequate total water intakes of about 2.5 litres for men and 2.0 litres for women, and US guidance gives higher figures of roughly 3.7 and 2.7 litres, all including food. Check current guidance from your health authority for your situation.