Most adults need between 2.7 and 3.7 litres of total water per day. Women generally need about 2.7 litres, while men need about 3.7 litres. These figures from the National Academies of Sciences, Engineering, and Medicine cover all water sources: plain water, other drinks, and the water naturally present in food.
That total is higher than many people expect, partly because roughly 20% of your daily water comes from food. Fruits, vegetables, soups, and even bread all contain water. So the amount you actually need to drink as liquid is lower than the headline number: closer to 2.2 litres for women and 3 litres for men in a typical day.
Why the “8 Glasses a Day” Rule Is Wrong
The advice to drink eight 8-ounce glasses (about 1.9 litres) of water per day is one of the most repeated health tips in existence, and it has no scientific basis. Michigan Medicine has noted there is no medical evidence that this specific amount is beneficial or that it helps with weight loss, despite being popularized by a well-known diet program decades ago. For most men, 1.9 litres of drinking water would actually fall short. For smaller, less active women, it might be more than necessary.
The real number depends on your body size, activity level, climate, and diet. A blanket rule was never going to work.
How Exercise Changes Your Needs
Physical activity dramatically increases the amount of water your body loses through sweat. During vigorous exercise, you can lose up to about 1.9 litres of fluid per hour. Endurance activities like distance running, intense hiking, or cycling can push losses even higher, up to roughly 2.8 litres per hour.
A practical guideline for adults during exercise is to drink about 180 to 350 millilitres every 20 minutes of activity. If you’re exercising for more than an hour, especially in the heat, you also lose significant amounts of salt, so a drink with electrolytes can be more effective than plain water. The key point is that a one-hour workout on a hot day could easily add a full litre or more to your daily needs.
Pregnancy and Breastfeeding
Pregnant women need somewhat more water than the standard recommendation, generally around 3 litres of total daily intake. The increase is more dramatic during breastfeeding. Nursing mothers need about 3.8 litres (roughly 16 cups) of total water per day to compensate for the extra fluid used to produce breast milk. If you’re breastfeeding and feeling constantly thirsty, that’s your body telling you something real: you’re manufacturing a liquid product, and the raw materials have to come from somewhere.
Hot Weather and High Altitude
Heat and humidity increase sweat rates even when you’re not exercising, so your baseline needs climb in summer or in tropical climates. There’s no single number that applies to every hot environment, but adding 0.5 to 1 litre on very warm days is a reasonable starting point.
Altitude is a less obvious factor. At high elevations, the air is drier, and you breathe faster to compensate for lower oxygen levels. Research published in Frontiers in Physiology found that respiratory water loss at very high altitude (around the pressure found at the top of an extreme mountain) was roughly 2.5 times greater than at sea level. The absolute numbers through the lungs alone are modest, but combined with increased breathing rate during exertion, altitude can meaningfully raise your daily fluid needs. If you’re hiking or skiing above 2,500 metres, drink more than you would at home.
How to Tell If You’re Drinking Enough
Rather than obsessing over exact litres, urine colour is the simplest and most reliable way to gauge your hydration. A pale, straw-like yellow means you’re well hydrated. Slightly darker yellow suggests you need more fluid. Medium to dark yellow, especially if the volume is low and the smell is strong, signals real dehydration.
A few caveats: B vitamins can turn urine bright yellow even when you’re perfectly hydrated, and certain foods like beetroot can change its colour. Medications can have similar effects. If you’re taking a supplement and your urine looks neon, colour alone won’t tell you much. In those cases, pay attention to how often you’re going. Urinating every two to four hours with a reasonable volume is a good sign.
The Real Benefits of Staying Hydrated
Adequate hydration supports kidney function, digestion, joint lubrication, temperature regulation, and cognitive performance. One of the most concrete, well-studied benefits involves kidney stones. Observational studies show that increasing fluid intake reduces stone recurrence rates by 50 to 60%. Clinical guidelines recommend drinking enough to produce more than 2.5 litres of urine daily for people at risk, but the research also shows that even partial increases in fluid intake, well below that target, cut recurrence risk significantly. The relationship between water intake and stone risk is continuous: more fluid generally means lower risk, with no sharp cutoff.
Mild dehydration, even just 1 to 2% of body weight in fluid loss, can impair concentration, increase headache frequency, and make you feel fatigued. Many people who describe an afternoon energy dip are simply not drinking enough during the first half of the day.
Can You Drink Too Much?
Yes. Water intoxication is rare but dangerous. It happens when you drink so much water so quickly that your kidneys can’t excrete it fast enough, diluting the sodium in your blood to dangerously low levels. According to Cleveland Clinic, symptoms can develop after drinking roughly 3 to 4 litres in just one to two hours. A safe upper limit is about 1 litre per hour. Beyond that, your kidneys start to fall behind.
This is most commonly seen in endurance athletes who drink aggressively during long events without replacing electrolytes, or in extreme water-drinking challenges. For most people, the risk is negligible during normal daily life. Just don’t try to “catch up” on a full day’s water in one sitting.
A Practical Daily Target
If you want a simple number to aim for, drinking about 2 litres of water per day is a reasonable baseline for most women, and about 2.5 to 3 litres for most men. The rest of your daily total will come from food and other beverages like tea, coffee, and milk (all of which count toward hydration despite old myths to the contrary).
Increase that amount if you exercise, live in a hot or dry climate, are pregnant or breastfeeding, or spend time at high altitude. Decrease it if you’re small-framed, sedentary, or eating a diet rich in water-heavy foods like fruits, soups, and salads. Your urine colour will tell you more than any formula. Pale yellow and frequent: you’re doing fine.