Short answer: In Europe, EFSA sets 2.0 liters (68 fl oz) of total water a day for women and 2.5 L (85 fl oz) for men; the US National Academies say about 2.7 L and 3.7 L. About 20% comes from food, so that means roughly 1.6–2.2 L (6–9 glasses) from drinks for women and 2.0–3.0 L (8–12 glasses) for men. Drink more in heat, when exercising or ill, and when pregnant or breastfeeding.
You lose water all day through urine, sweat and breath, and replace it with drinks, food and a little water your metabolism makes. Here is what the official numbers mean in glasses, how per-kilogram rules work, and when you need more.
If you have heart failure or kidney disease, take diuretics (water tablets) or have been told to limit fluids, follow your doctor’s fluid advice rather than any number here. The same applies if a doctor has told you to drink more.
How much water should you drink a day?
Reference values are for total water: everything you drink plus the water in food. EFSA, the European Food Safety Authority, considers 2.0 liters a day adequate for women and 2.5 L for men, from age 14 on. The US National Academies found that adequately hydrated women take in about 2.7 L a day and men about 3.7 L. About 80% comes from drinks, including caffeinated ones, and 20% from food, the National Academies add.
| Who | EFSA: total water | EFSA: from drinks (about 80%) | US: total water | US: from drinks |
|---|---|---|---|---|
| Women | 2.0 L | 1.6 L (6–7 glasses) | 2.7 L | 2.2 L (about 9 glasses) |
| Men | 2.5 L | 2.0 L (8 glasses) | 3.7 L | 3.0 L (12 glasses) |
| Pregnancy | 2.3 L | 1.8 L (about 7 glasses) | 3.0 L | 2.3 L (about 9 glasses) |
| Breastfeeding | 2.7 L | 2.2 L (about 9 glasses) | 3.8 L | 3.1 L (about 12 glasses) |
EFSA’s values assume a moderate climate and moderate activity, so treat them as a range, not a target. The National Academies concluded that the vast majority of healthy people meet their needs by letting thirst be their guide, and the NHS advises drinking enough that your pee is a clear pale yellow.
Do you need to drink 8 glasses of water a day?
The “8 × 8” rule – eight 8-ounce glasses, about 1.9 L (64 fl oz) – is easy to remember, but it isn’t evidence-based. A 2002 review in the American Journal of Physiology found no scientific studies supporting it. A 2022 study in Science of 5,604 people in 26 countries concluded that the rule is not backed by objective evidence and that one size does not fit all.
Still, 8 glasses is in the right range for many adults: the NHS uses 6 to 8 cups or glasses of fluid a day as a guide, and they don’t all have to be plain water.
How much water should you drink by body weight?
EFSA and the US give no per-kilogram value. The reference values of the German, Austrian and Swiss nutrition societies (D-A-CH), published by the German Nutrition Society (DGE), do: 35 ml per kilogram of body weight a day at ages 19–50 and 30 ml/kg from 51. That is water from drinks and food. In the same table, a bit over 60% comes from drinks: 1,410 ml of 2,270 ml at ages 25–50.
| Body weight | Ages 19–50 (35 ml/kg) | Of which drinks (about 60%) | Age 51+ (30 ml/kg) |
|---|---|---|---|
| 50 kg (110 lb) | 1.75 L | 1.1 L | 1.5 L |
| 60 kg (132 lb) | 2.1 L | 1.3 L | 1.8 L |
| 70 kg (154 lb) | 2.45 L | 1.5 L | 2.1 L |
| 80 kg (176 lb) | 2.8 L | 1.7 L | 2.4 L |
| 90 kg (198 lb) | 3.15 L | 1.9 L | 2.7 L |
| 100 kg (220 lb) | 3.5 L | 2.1 L | 3.0 L |
Worked example: at 70 kg (154 lb) and age 35, 70 × 35 ml = 2,450 ml from drinks and food. About 60% of that, 1.5 L (51 fl oz), comes from drinks: six 250 ml glasses. That matches the DGE’s everyday advice of around 1.5 liters of drinks a day.
- It includes food. Many online calculators call the 30–35 ml/kg result drinking water; in the D-A-CH table it is drinks plus food.
- Weight is only a proxy. Needs follow energy turnover, the DGE notes; activity, heat and humidity shift them too.
- It climbs fast. At 100 kg (220 lb) it gives 3.5 L, well above EFSA’s 2.5 L for men. Don’t force down a calculated number.
When do you need more water?
In extreme heat and with hard exercise, water losses can reach about 8 liters a day, EFSA notes, and the salts lost in sweat need replacing too.
Hot weather
In the Science study, water turnover was about 1 L a day higher at 30 °C (86 °F) than at 0–10 °C (32–50 °F). For work in the heat, NIOSH, the US occupational health institute, advises 1 cup (8 fl oz) of water every 15–20 minutes during moderate work and generally no more than 6 cups (about 1.4 L) an hour.
Exercise
The National Athletic Trainers’ Association advises keeping fluid loss during exercise below 2% of body weight, not gaining weight (a sign of drinking too much), and replacing 100–150% of the loss afterwards. Weigh yourself before and after a session: each kilogram (2.2 lb) lost is roughly a liter of sweat, so 0.8 kg (1.8 lb) less after a run means drinking about 0.8–1.2 L (27–41 fl oz) over the next few hours. Without a known sweat rate, drinking to thirst is a safe way to avoid overdrinking. For walks, see calories burned walking.
Pregnancy and breastfeeding
EFSA adds 300 ml a day in pregnancy and about 700 ml while breastfeeding. The US values are 3.0 L and 3.8 L of total water.
Fever, vomiting or diarrhea
Fever, vomiting and diarrhea raise water needs, the DGE notes. If you feel sick, the NHS suggests small sips, building up gradually, and a pharmacist can recommend an oral rehydration solution. Get urgent advice if you feel unusually tired or dizzy on standing, or pass little or dark yellow pee.
Older age, fasting and medication
EFSA keeps the adult values for older people, as ageing kidneys concentrate urine less well, and the NHS counts older adults among those at higher risk of dehydration. In Ramadan, see how to stay hydrated between iftar and suhoor; on GLP-1 medication, see fiber and fluids.
Do coffee and tea count? Drinks and water-rich foods
Yes. The National Academies concluded that caffeinated drinks contribute to daily water intake much like non-caffeinated ones. The limit is caffeine itself: EFSA considers up to 400 mg a day safe for healthy adults and 200 mg in pregnancy and while breastfeeding. A 200 ml cup of filter coffee has about 90 mg, a 220 ml cup of black tea about 50 mg. When you are already dehydrated, the NHS advises avoiding caffeinated drinks and alcohol.
Water, milk and unsweetened tea or coffee are the best everyday choices; the NHS limits fruit juice and smoothies to 150 ml (5 fl oz) a day. Food counts too. Values below are rounded from USDA FoodData Central; 1 g of water is about 1 ml.
| Food | Water per 100 g | Typical portion | Water per portion |
|---|---|---|---|
| Cucumber with peel, raw | 95 g | 100 g | 95 g |
| Tomato, raw | 95 g | 1 cup chopped, 180 g | 170 g |
| Chicken noodle soup, canned, made with water | 94 g | 1 cup, 248 g | 234 g |
| Watermelon | 91 g | 1 cup diced, 152 g | 139 g |
| Plain whole-milk yogurt | 88 g | 170 g | 149 g |
| Orange | 87 g | 1 medium, 131 g | 114 g |
| Apple, with skin | 86 g | 1 medium, 182 g | 156 g |
| White rice, cooked | 68 g | 1 cup, 158 g | 108 g |
A tomato, a bowl of soup, an orange and an apple together add about 670 ml (23 fl oz) of water.
Signs you’re drinking too little, or too much
The NHS lists these symptoms of dehydration: thirst; dark yellow, strong-smelling pee and peeing less often; headache, dizziness or lightheadedness; tiredness; a dry mouth, lips and tongue; and sunken eyes. Aim for pee that is clear pale yellow, judged in a clear container rather than in the toilet bowl, as the athletic trainers’ statement suggests.
Too much water is rare in healthy people, but possible. Healthy kidneys can excrete at most about 0.7–1.0 L (24–34 fl oz) an hour, EFSA notes. Drinking well beyond that, or a lot of fluid while losing salt in sweat, can dilute the sodium in your blood (hyponatremia), which in severe cases makes the brain swell. Exercise-related cases occur mainly in long endurance events, the National Academies note.
Does drinking water help you lose weight?
A little, perhaps, as part of a diet. A 2013 systematic review found that in people on a weight-loss or maintenance program, drinking more water reduced weight more than the program alone, while general-population studies were inconsistent; the evidence is low. In one trial, 48 adults aged 55–75 on a calorie-reduced diet who drank 500 ml (17 fl oz) of water before each meal lost about 2 kg (4.4 lb) more over 12 weeks.
The clearer gain is what water replaces. A 12 fl oz (355 ml) can of regular cola has about 155 kcal, according to USDA data, so swapping one a day for water saves about 1,085 kcal a week. Water still works only through your overall calorie deficit.
How Portiva helps
Portiva’s Today screen has a water bar: one tap adds 250 ml (8.5 fl oz), and you can undo the last addition. You set your own daily water goal in Settings, for example the one your doctor gave you. Water tracking is free in every plan.
With your permission, the water you log is saved to Apple Health, and undoing an entry removes it there too.
Frequently asked questions
Is 2 liters of water a day enough?
For many adults, yes. EFSA considers 2.0 L of total water a day adequate for women and 2.5 L for men, and about a fifth of total water comes from food. If you drink 2 L and also eat fruit, vegetables or soup, you reach those values. Drink more in heat, during exercise or when ill.
Is it bad to drink 3 or 4 liters of water a day?
For a healthy adult who spreads it through the day, usually not: the US value for men is 3.7 L of total water, and healthy kidneys pass the extra as dilute urine. The risk is drinking a lot very fast, beyond the 0.7–1.0 L an hour EFSA gives as the kidneys’ maximum. With heart or kidney disease, follow your doctor’s advice.
Does sparkling water count?
Yes, it counts like still water. The NHS suggests sparkling water, or a slice of lemon or lime, if you don’t like plain water. Check flavored waters: the DGE warns that some contain sugar.
Should you drink water even if you’re not thirsty?
Most healthy people can let thirst guide them, the National Academies concluded, helped by drinks with meals. Thirst can lag behind your needs in hard training, so drink to a plan in long sessions and in the heat.
Sources
- Dietary Reference Values for nutrients: Summary report (section 3.5, Water), European Food Safety Authority (2017).
- Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate: Water, National Academies (2005).
- Report sets dietary intake levels for water, salt, and potassium, National Academies (2004).
- Referenzwerte: Wasser, Deutsche Gesellschaft für Ernährung (DGE).
- Getränke, Deutsche Gesellschaft für Ernährung (DGE).
- Water, drinks and hydration, NHS (2023).
- Dehydration, NHS (2026).
- “Drink at least eight glasses of water a day.” Really?, Valtin H, American Journal of Physiology (2002).
- Variation in human water turnover associated with environmental and lifestyle factors, Yamada Y et al., Science (2022).
- National Athletic Trainers’ Association position statement: fluid replacement for the physically active, McDermott BP et al., Journal of Athletic Training (2017).
- Heat stress: recommendations, NIOSH, Centers for Disease Control and Prevention.
- Caffeine, European Food Safety Authority (2015).
- Association between water consumption and body weight outcomes: a systematic review, Muckelbauer R et al., American Journal of Clinical Nutrition (2013).
- Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults, Dennis EA et al., Obesity (2010).
- FoodData Central (SR Legacy data), U.S. Department of Agriculture, Agricultural Research Service (2018).