Most adults need between 2.7 and 3.7 litres of total water per day. Women should aim for about 2.7 litres, and men about 3.7 litres. These figures, set by the National Academies of Sciences, Engineering, and Medicine, include all water sources: plain water, other beverages, and the water naturally present in food.
What the Numbers Actually Mean
The 2.7 and 3.7 litre targets cover everything you consume that contains water. That includes coffee, tea, juice, soup, and water-rich foods like fruits and vegetables. In the U.S., plain drinking water accounts for roughly one-third of total water intake, with the rest coming from food and other beverages. So if you’re a woman aiming for 2.7 litres total, you don’t necessarily need to drink 2.7 litres of plain water. You likely need closer to 1.5 to 2 litres from your glass or bottle, with the remainder covered by meals and other drinks.
For men, that translates to roughly 2.5 to 3 litres of actual drinking water per day, depending on diet. People who eat a lot of soups, salads, and fresh fruit get more water from food than those who eat mostly dry, processed meals.
The “8 Glasses a Day” Rule Has No Scientific Basis
The popular advice to drink eight glasses of eight ounces of water (about 1.9 litres) each day has no research behind it. A thorough review published in the American Journal of Physiology found no scientific studies supporting the 8×8 rule. The recommendation likely traces back to a misreading of a 1945 statement from the Food and Nutrition Board, which mentioned 2.5 litres of daily water intake but noted that most of it already comes from food. That last detail was ignored, and the simplified “eight glasses” version stuck.
This doesn’t mean the rule is harmful. For many people, drinking about 2 litres of water a day is a reasonable target that falls within the broader guidelines. It’s just not a magic number backed by evidence.
How Exercise Changes Your Needs
Physical activity increases your water needs significantly. Sweat rates vary widely, from about 0.5 litres per hour during light activity to over 2.5 litres per hour during intense exercise in the heat. The National Athletic Trainers’ Association recommends drinking 200 to 300 millilitres every 10 to 20 minutes during exercise, which works out to roughly 0.6 to 1.8 litres per hour depending on intensity and conditions.
A practical approach: weigh yourself before and after a workout. Every kilogram lost represents roughly one litre of fluid you need to replace. If you’re consistently losing more than 2% of your body weight during exercise (about 1.5 kg for a 75 kg person), you’re not drinking enough during the session.
Hot Weather and Altitude
Heat and humidity force your body to sweat more to cool down, increasing water losses. There’s no single threshold where you need to bump up intake, but any time temperatures climb above what you’re used to, your fluid needs rise. This applies even if you’re not exercising. Sitting outside on a hot day still increases sweat output compared to a climate-controlled room.
Altitude has a similar effect. At higher elevations, you lose more water through breathing because the air is drier, and your body initially produces more urine as it adjusts. If you’re traveling to a hot climate or a high-altitude destination, adding an extra 0.5 to 1 litre per day is a reasonable starting point.
Pregnancy and Breastfeeding
Pregnant women are generally advised to increase total water intake to about 3 litres per day. During breastfeeding, that number jumps further. The Academy of Nutrition and Dietetics recommends nursing mothers get about 3.8 litres of total water daily (roughly 16 cups) to compensate for the fluid used to produce milk. As with the general guidelines, this total includes water from food and all beverages.
Older Adults and Reduced Thirst
Adults over 70 have the same baseline recommendation as younger adults: 3.7 litres for men and 2.7 litres for women. But meeting that target gets harder with age. Older adults lose some of their thirst sensation, so they’re less likely to feel the urge to drink even when their body needs fluid. This risk increases if they’re taking medications that cause fluid loss, like certain blood pressure drugs. Setting a schedule or keeping a water bottle visible throughout the day can help compensate for a muted thirst signal.
How to Tell If You’re Drinking Enough
Rather than obsessing over an exact litre count, your body gives you reliable feedback. Urine color is the simplest gauge. Pale, straw-colored urine with little odor signals good hydration. As the color darkens toward amber or honey, you’re moving into mild to moderate dehydration. Very dark urine in small amounts, especially with a strong smell, means you need to drink water promptly.
Frequency matters too. Most well-hydrated people urinate six to eight times per day. If you’re going significantly less often, or if your urine is consistently dark by midday, you’re likely falling short.
Can You Drink Too Much?
Yes. Drinking large volumes of water too quickly can dilute the sodium in your blood, a condition called hyponatremia. According to Cleveland Clinic, symptoms can develop after drinking 3 to 4 litres in just an hour or two. A safer upper limit is about 1 litre per hour. Your kidneys can process roughly 0.8 to 1 litre of water per hour, so spreading your intake throughout the day is far more effective than chugging large amounts at once.
Hyponatremia is rare in everyday life but does occur in endurance athletes and people who force themselves to drink extreme amounts of water. Symptoms include nausea, headache, confusion, and in severe cases, seizures. Drinking to thirst rather than following an aggressive schedule is the best prevention.
A Practical Daily Target
For most adults in a temperate climate with moderate activity levels, these drinking water targets work well as a starting point:
- Women: about 1.5 to 2 litres of water and other beverages per day
- Men: about 2.5 to 3 litres of water and other beverages per day
Add 0.5 to 1 litre for every hour of moderate to vigorous exercise, more in hot conditions. Adjust upward during pregnancy or breastfeeding. And pay attention to what your body tells you: thirst, urine color, and energy levels are more useful day-to-day indicators than any fixed number.