Caffeine vs WaterHydration
How Much Water Should You Drink a Day? (It's Not Always 8 Glasses)
The "8 glasses a day" rule has no solid science behind it. Here's what the official guidelines say, how to work out your own daily water goal from your weight and activity, and whether coffee counts.
The short answer
- The U.S. National Academies suggests about 2.7 litres (91 oz) a day for women and 3.7 litres (125 oz) for men, counting all drinks and food.
- About 80% of that comes from drinks, so plan on roughly 2.2 L (women) to 3 L (men) from what you drink.
- A simple personal estimate: about 33 ml per kg of body weight (0.5 oz per lb), plus more for exercise and heat.
- Coffee, tea and other drinks count. "8 glasses of water" was never based on evidence.
Few pieces of health advice are as famous as "drink eight glasses of water a day". Few are as poorly supported. When a kidney physiologist went looking for the evidence behind it in 2002, he couldn't find any. Your body does need water, but how much depends on your size, how active you are, the weather and what else you eat and drink.
What the official guidelines say
In 2004, the Institute of Medicine (now the National Academies) looked at national data and found that adequately hydrated women take in about 2.7 litres of total water a day and men about 3.7 litres. Those became the "adequate intake" values. Two details are often left out:
- They include water from food, which provides around 20%. Fruit, vegetables, soup and yoghurt are mostly water.
- They include all drinks, "including caffeinated beverages".
The panel was explicit that it didn't offer a glasses-per-day rule, and that most healthy people meet their needs by drinking with meals and letting thirst guide them. People who are very active or live in hot climates may need more.
Work out your own daily water goal
Bigger bodies need more water, so a weight-based estimate is more personal than one number for everyone. A common starting point is 30–35 ml per kg. Our app uses 33 ml/kg, then adds for activity and heat, and keeps the goal between 1.5 and 5 litres:
| Body weight | Base goal (33 ml/kg) | Moderately active (+350 ml) | Very active (+700 ml) |
|---|---|---|---|
| 50 kg (110 lb) | 1.65 L | 2.0 L | 2.35 L |
| 60 kg (132 lb) | 2.0 L | 2.35 L | 2.7 L |
| 70 kg (154 lb) | 2.3 L | 2.65 L | 3.0 L |
| 80 kg (176 lb) | 2.65 L | 3.0 L | 3.35 L |
| 90 kg (198 lb) | 3.0 L | 3.3 L | 3.7 L |
| 100 kg (220 lb) | 3.3 L | 3.65 L | 4.0 L |
Add about 500 ml on hot days. You'll also need more if you have a fever, vomiting or diarrhoea, or you're pregnant or breastfeeding. Treat these as targets for a normal day, not a rule to force down: if you're rarely thirsty and your urine is pale, you're probably doing fine.
Signs you're not drinking enough
- Feeling thirsty, or a dry mouth
- Dark yellow urine, or peeing less than usual
- Headache, tiredness or trouble concentrating
- Dizziness when you stand up
Even mild dehydration matters. In one study, young women who lost about 1.4% of their body weight in fluid (about 850 g for a 60 kg person) reported worse mood, lower concentration and more headaches. The CDC notes dehydration can cause unclear thinking, mood changes, overheating, constipation and kidney stones.
Does coffee count toward your water intake?
Yes. This surprises a lot of people. Coffee is a mild diuretic, but for regular drinkers a normal cup hydrates about as well as water. In a 2014 study, four cups of coffee a day kept people as hydrated as four cups of water. Only big single doses, around 500 mg of caffeine, noticeably increase urine. The full story is in does coffee dehydrate you?
That said, plain water is still the best default drink: it has no calories, no sugar and no caffeine, so it never keeps you up at night.
Easy ways to drink more water
- Pair it with habits you already have. A glass with every meal, and one alongside each coffee.
- Keep a bottle in sight at your desk, in your bag and in the car.
- Front-load your day. Drink more in the morning and early afternoon so you're not up at night.
- Eat water-rich foods like fruit, salad and soup.
- Track it for a week. Most people are surprised by how little, or how much, they actually drink.
Can you drink too much water?
Yes, though it's rare. Drinking very large amounts quickly, especially during long endurance events, can dilute the sodium in your blood (hyponatraemia), which can be dangerous. For everyday life, spreading your water through the day and following thirst keeps you safe. This is one reason our app caps its daily goal at 5 litres.
Frequently asked questions
How much water should I drink a day?
The National Academies suggests about 2.7 litres a day for women and 3.7 litres for men from all drinks and food. About 80% usually comes from drinks. A personal estimate is roughly 33 ml per kg of body weight, plus more for exercise and hot weather.
Is 8 glasses of water a day right?
It's a reasonable rough target for some people, but it isn't based on evidence. A 2002 scientific review found no studies supporting it, and it ignores fluid from food and other drinks.
How much water should I drink based on my weight?
Multiply your weight in kg by about 33 ml (or your weight in pounds by about 0.5 oz). A 70 kg person would aim for about 2.3 litres, plus 350–700 ml if active and about 500 ml more in hot weather.
Does coffee count as water?
Yes. Coffee and tea count toward your total daily fluid. Moderate coffee hydrates regular drinkers about as well as water.
What's the best time to drink water?
Spread it through the day, with a glass at each meal and when you're thirsty. Drinking more earlier in the day helps you avoid waking up at night to pee.
Sources
- National Academies. Report sets dietary intake levels for water, salt, and potassium, 2004.
- Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 × 8"? Am J Physiol, 2002.
- Mayo Clinic. Water: How much should you drink every day? 2026.
- CDC. About water and healthier drinks, 2026.
- Armstrong LE, et al. Mild dehydration affects mood in healthy young women. J Nutr, 2012.
- Killer SC, et al. No evidence of dehydration with moderate daily coffee intake. PLoS One, 2014.
General wellness information, not medical advice. If you're pregnant, have a heart condition, or take medication that affects caffeine, check your limits with a doctor.