How Much Water Should You Drink a Day? An Evidence-Based Guide

How much water do you need per day?

There is no single number that fits everyone, but official reference values give a good anchor. The European Food Safety Authority (EFSA, 2010) set adequate intakes of total water of 2.0 litres a day for adult women and 2.5 litres for adult men, assuming moderate temperatures and moderate activity. The US National Academies (2005) set higher values of 2.7 L for women and 3.7 L for men.

Both figures are total water: plain water, every other drink and the water in food. They are also adequate intakes, which means they are based largely on what healthy people actually consume rather than on a strict minimum. That is why the European and American numbers differ, and why your own needs may be lower or higher.

The popular advice of eight glasses a day (about 2 litres) is not based on a specific study, but it is not far from these values once you count what you eat. What matters more than hitting an exact number is adjusting for your body size, activity and climate.

A simple rule of thumb: about 35 ml per kg

A common way to personalise intake is to multiply your body weight by about 35 ml. Our water intake calculator uses exactly this base, then adds about 350 ml for every 30 minutes of exercise and 500 ml for hot weather. To avoid extreme results, it never goes below 1.5 litres or above 5 litres a day.

Treat the result as a total-water estimate, not a strict drinking target. Roughly 20% of daily water usually comes from food, so if you eat plenty of fruit, vegetables, soups and dairy, you will need to drink a little less. Because the rule scales with body weight, it can also overshoot for people with a very large body size, so let thirst and urine color guide the final amount.

• 60 kg: about 2.1 L of total water on a quiet day.
• 70 kg: about 2.5 L.
• 80 kg: about 2.8 L.
• 70 kg with a 60-minute workout on a hot day: about 2.5 + 0.7 + 0.5, so roughly 3.7 L.

Exercise and hot weather: how much extra?

Sweat losses vary enormously between people and conditions, from a few hundred millilitres to more than 2 litres per hour in hard training in the heat. The American College of Sports Medicine (ACSM) position stand on fluid replacement recommends drinking enough to avoid losing more than about 2% of body weight from water loss during exercise, while also avoiding weight gain from drinking too much.

The easiest way to learn your own needs is to weigh yourself before and after a typical session, without clothes and after using the toilet. Each kilogram lost is roughly one litre of fluid. If you regularly lose more than 2% of your weight, drink a bit more during similar sessions; if you gain weight, you are drinking more than you need.

• Before: drink normally through the day; a glass or two in the hours before training is enough for most people.
• During: for sessions under an hour, water when thirsty is usually fine.
• Long or very sweaty sessions (more than 1–2 hours, or in heat): a drink with sodium and carbohydrate can help replace what you lose.
• After: if you need to recover quickly, the ACSM suggests about 1.5 L of fluid for every kg lost, spread over the following hours and taken with meals or salty snacks.

Check your hydration: urine color and thirst

You don't need lab tests to keep an eye on hydration. In a classic 1994 study, Armstrong and colleagues found that urine color tracked laboratory markers of urine concentration well enough to be a practical field tool. Pale yellow or straw-colored urine usually means you're well hydrated, while dark yellow or amber suggests you need more fluid.

Urine that is completely clear all day long may mean you're drinking more than you need. Keep in mind that first-morning urine is naturally darker, B vitamins can turn it bright yellow, and some foods and medications change its color. For most healthy people in daily life, thirst plus drinking with meals keeps hydration in a normal range, as the National Academies note; thirst can be less reliable in older adults, who may need to drink more deliberately.

Can you drink too much water?

Yes. Drinking much more than your body loses can dilute the sodium in your blood, a condition called hyponatremia. It is uncommon in daily life but has been seen in marathon runners, hikers and people taking part in water-drinking challenges. The 2015 international consensus on exercise-associated hyponatremia identified drinking beyond sweat and urine losses as the main cause and recommended using thirst to guide drinking during exercise.

Early symptoms such as nausea, headache, bloating and puffiness can look like dehydration, which is why forcing down more water can make things worse. Severe cases cause confusion and seizures and are a medical emergency.

• Don't force yourself to drink large volumes quickly, especially in a contest or on a dare.
• Gaining weight during a long event is a clear sign of overdrinking.
• In events lasting several hours, drink to thirst and include some sodium from food or sports drinks.

Water from food and other drinks counts

Plain water is the best default because it has no calories or sugar, but it is not the only source. According to the National Academies, drinks provide around 80% of total water intake and food the remaining 20% or so. Fruit and vegetables such as cucumber, lettuce, tomatoes, watermelon and oranges are more than 85–90% water, and soups, milk and yogurt add a meaningful amount too.

Coffee and tea also count. In people who drink them regularly, the mild diuretic effect of caffeine is small, so they still add to your daily total. Juice, soft drinks and sweetened coffee drinks hydrate as well but add calories and sugar, and alcohol increases urine output, so it's better not to rely on it for fluid.

When to talk to a doctor about fluid intake

General rules like 35 ml per kg are meant for healthy adults. Some conditions change how much fluid is safe. People with heart failure, advanced kidney disease, liver cirrhosis or certain hormonal disorders are often told to limit fluids, while others, such as people prone to kidney stones, may be advised to drink more. Diuretics and some other medications also affect fluid balance.

Needs rise during pregnancy and breastfeeding: EFSA suggests about 300 ml a day extra during pregnancy and 600–700 ml extra while breastfeeding. If you have any of these conditions, take regular medication or notice unusual thirst or changes in how much you urinate, ask your doctor what daily amount is right for you rather than following a calculator.