As a rule of thumb for how much water you should drink a day, most healthy adults need about 2 to 3 liters (68 to 101 fl oz) of fluid from drinks, which is roughly 35 mL per kilogram of body weight. Add about 350 mL (12 oz) for every 30 minutes of exercise and a little more in hot weather.
The water intake calculator does that arithmetic for your weight, activity, climate and pregnancy status in a few seconds. The rest of this guide explains where the numbers come from, why “eight glasses a day” is only half right, how to adjust for exercise and heat, and how to tell if you are getting too little or, less often, too much.
Where “8 glasses a day” came from
The familiar 8 × 8 rule (eight 8-ounce glasses, or 64 fl oz, about 1.9 liters) has no clear scientific origin. It is often traced to a 1945 US Food and Nutrition Board recommendation of about 2.5 liters of water a day for adults. The next sentence of that recommendation, which tends to get left out, said that most of this quantity is already contained in prepared foods.
So the original advice was about total water from all sources, not glasses of plain water on top of everything else. Eight glasses is not a bad target for many people, but it is neither a minimum nor a magic number.
What the official guidelines say
The most widely cited figures come from the US National Academies, which set an adequate intake for total water from food and drinks combined. The European Food Safety Authority (EFSA) uses somewhat lower figures:
| US total water (food + drinks) | About 80% from drinks | EFSA total water | |
|---|---|---|---|
| Men | 3.7 L (125 fl oz) | about 3.0 L, or 13 cups | 2.5 L |
| Women | 2.7 L (91 fl oz) | about 2.2 L, or 9 cups | 2.0 L |
| Pregnancy | 3.0 L | about 2.3 L | +0.3 L |
| Breastfeeding | 3.8 L | about 3.1 L | +0.7 L |
These are not requirements. They are the median intakes of healthy populations who were not dehydrated, so they describe what normal looks like. Your own needs can be higher or lower depending on your size, activity and environment, and the panel noted that for most people thirst and normal drinking with meals keep them in balance.
How much water should I drink a day for my weight?
A body-weight rule personalizes things better than one number for everyone. A common starting point, and the one our calculator uses, is about 35 mL per kilogram (roughly 0.5 fl oz per pound) from drinks each day:
| Body weight | Baseline fluid | In fl oz | In 8-oz cups | With 30 min exercise |
|---|---|---|---|---|
| 50 kg (110 lb) | 1.75 L | 59 | 7.4 | 2.1 L |
| 60 kg (132 lb) | 2.1 L | 71 | 8.9 | 2.45 L |
| 70 kg (154 lb) | 2.45 L | 83 | 10.4 | 2.8 L |
| 80 kg (176 lb) | 2.8 L | 95 | 11.8 | 3.15 L |
| 90 kg (198 lb) | 3.15 L | 107 | 13.3 | 3.5 L |
| 100 kg (220 lb) | 3.5 L | 118 | 14.8 | 3.85 L |
Bigger bodies hold and lose more water. A 70 kg man carries about 40 liters of water in total, which you can estimate for yourself with the total body water calculator. For people with a lot of body fat, the per-kilogram rule can overshoot slightly, because fat tissue holds less water than muscle.
If you think in ounces or cups, the mL to oz converter and the mL to cups converter make it easy to translate a bottle size into your daily target.
Worked example: a full day’s target
Take a 60 kg (132 lb) woman who runs for 45 minutes most days and lives somewhere with hot summers.
- Baseline: 60 × 35 mL = 2,100 mL.
- Exercise: 45 ÷ 30 × 350 mL = 525 mL. Running total: 2,625 mL.
- Hot weather: add 10%, so 2,625 × 1.1 = 2,888 mL, or about 2.9 L (98 fl oz, about 12 cups).
On a cool rest day the same woman needs only about 2.1 L. That gap is why a fixed daily rule rarely fits. For comparison, a 70 kg (154 lb) man who exercises for 30 minutes in a temperate climate comes out at 2,450 + 350 = 2,800 mL, or 2.8 L (about 12 cups).
In practice she could cover that with a 500 mL glass on waking, a drink with each of three meals, a 750 mL bottle during and after her run, and tea or water through the afternoon. Nothing about it needs to be forced.
What counts toward your daily water
Food
Fruit and vegetables are 80 to 95% water. Soups, yogurt, cooked rice and pasta also contribute. On average food supplies about 20% of total water, more if you eat plenty of produce.
Coffee, tea and other drinks
Coffee and tea count. Caffeine has a mild diuretic effect in people who are not used to it, but regular drinkers adapt, and studies show moderate coffee hydrates about as well as water. Keep total caffeine under about 400 mg a day for most adults (200 mg in pregnancy); the caffeine calculator adds up what you are actually having. Milk, juice and soft drinks also count for fluid, though sugary drinks add calories, which the guide on how many calories you should eat helps you keep track of.
Alcohol
Alcohol does increase urine output, especially in stronger drinks, so beer and wine count for less than their volume and spirits can cause a net loss. Alternate with water.
Adjusting for exercise, heat and life stages
Exercise
Sweat rates vary from about 0.5 L to more than 2 L per hour depending on intensity, temperature and genetics. The easiest way to know yours is to weigh yourself before and after a typical hour of exercise. If a 75 kg runner drinks 0.5 L during an hour-long run and finishes at 74.2 kg, the sweat loss is 0.8 + 0.5 = about 1.3 L per hour. Replace about 1.5 times the weight lost over the next few hours, here roughly 1.2 L, since you keep losing some fluid as urine.
For sessions over an hour, or in heat, water alone may not be enough: you also lose sodium. A homemade sports drink with a little sugar and salt is a cheap fix. Higher heart rates during long sessions are often a sign of fluid loss; the guide to heart rate zones explains this cardiac drift.
Heat, altitude and illness
Hot or humid weather, high altitude, fever, vomiting and diarrhea all raise fluid needs, sometimes by a liter or more a day. With vomiting or diarrhea, oral rehydration solutions replace salts better than plain water.
Pregnancy and breastfeeding
Add about 300 mL a day in pregnancy and 700 mL while breastfeeding. The guide on calculating your due date covers the pregnancy timeline itself.
Older adults
Thirst becomes less reliable with age, and some medicines (diuretics in particular) increase losses. Older adults often do better with a routine, such as a glass with every meal and medication, than with relying on thirst alone.
Signs you are drinking too little or too much
Under-hydration
- Dark yellow urine, or going many hours without needing to urinate
- Thirst, dry mouth and dry lips
- Headache, tiredness, dizziness on standing
- Exercise feeling harder than usual at the same pace
Urine color is the simplest daily check. Pale straw to light yellow is the goal. Note that B vitamins can turn urine bright yellow regardless of hydration.
Over-hydration
Drinking far more than you need is less common but can be dangerous. Healthy kidneys can clear roughly 0.8 to 1.0 L of water an hour. Drink much faster than that for several hours and blood sodium can fall, a condition called hyponatremia. Symptoms include nausea, headache, confusion and swelling, and severe cases are a medical emergency. It shows up most in endurance events where people drink on a fixed schedule regardless of thirst, and in water-drinking challenges. Clear, colorless urine all day long is a sign you can ease off.
The Mayo Clinic’s overview of water intake gives a good summary of both risks. This guide is general information, not personal medical advice. If you have heart failure, kidney disease or another condition where fluids are restricted, follow your doctor’s target rather than any general formula.
Quick tips for staying hydrated
- Drink with meals. Three meals and a couple of snacks with a drink each gets most people most of the way.
- Keep a bottle visible. A 750 mL bottle refilled three times is 2.25 L. The liters to gallons converter helps if your bottle is labeled in US units.
- Front-load your day. A glass on waking and more before lunch beats catching up at night and waking to use the bathroom.
- Eat your water. Fruit, vegetables and soups count.
- Weigh in for long sessions. A before-and-after weigh-in is the single best way to learn your sweat rate.
Frequently asked questions
How much water should I drink a day to lose weight?
The same amount as usual: roughly 35 mL per kg. Drinking a glass of water before meals may help you eat a little less, but water itself does not burn fat. A calorie deficit does that.
Is 3 liters of water a day too much?
For most adults, 3 liters spread through the day is safe, especially if you are large, active or in a hot climate. Problems arise when people drink several liters in a few hours, not across a whole day.
Does coffee dehydrate you?
Not in moderate amounts. Regular coffee drinkers retain most of the fluid, so coffee and tea count toward your daily intake.
How can I tell if I am drinking enough?
Pale yellow urine, urinating every 3 to 4 hours during the day and rarely feeling thirsty are good signs. Dark urine or headaches in the afternoon suggest you need more.
Do I need more water when I exercise?
Yes. Add about 350 mL (12 oz) for every 30 minutes of exercise as a starting point, more in heat, and weigh yourself before and after a session to find your personal sweat rate.
There is no single right amount of water for everyone, but there is a sensible range for you. Start with about 35 mL per kilogram, add for exercise and heat, count your food and other drinks, and use thirst and urine color to fine-tune. Plug your numbers into the water intake calculator once, then let your body do most of the work.