How Much Water Should You Drink a Day? What the Science Says
Eight glasses is a nice round number, but it isn't the whole story. Here's what experts actually recommend and how to tell if you're getting enough.
How much water should you drink a day? The US National Academies suggest an adequate daily fluid intake of about 3.7 litres (roughly 15.5 cups) for men and 2.7 litres (about 11.5 cups) for women. That figure includes all fluids and the water in food, which provides around 20% of the total. In practice, most healthy adults do well by drinking when thirsty, having water with meals and watching the colour of their urine.
This article is general information, not medical advice. If you have a heart, kidney or liver condition, are pregnant, or take medication that affects fluid balance, ask your doctor what’s right for you.
Where the “8 glasses a day” rule comes from
The popular advice to drink eight 8-ounce glasses of water a day is easy to remember, which is probably why it stuck. It’s not a bad starting point — it adds up to about 2 litres — but it isn’t based on a specific scientific requirement. It also ignores the fluid you get from tea, coffee, milk, soup, fruit and vegetables.
How much water you need depends on you
Your needs rise and fall with several factors:
- Activity. Exercise that makes you sweat increases your fluid needs, sometimes significantly.
- Climate. Hot or humid weather, and high altitude, raise the amount you lose.
- Body size. Larger people generally need more fluid.
- Health. Fever, vomiting and diarrhoea all increase fluid loss.
- Pregnancy and breastfeeding. Both raise daily fluid needs.
| Group | Approximate total daily fluid (all sources) |
|---|---|
| Adult men | About 3.7 L (15.5 cups) |
| Adult women | About 2.7 L (11.5 cups) |
| During pregnancy | About 3.0 L (10 cups from drinks) |
| While breastfeeding | About 3.8 L (13 cups from drinks) |
These are general reference values for healthy adults, not strict targets. The Mayo Clinic’s overview of daily water needs is a good, readable summary.
Signs you’re drinking enough water
- You rarely feel very thirsty.
- Your urine is pale yellow, like light straw.
- You urinate regularly through the day.
- You don’t get frequent headaches or feel lightheaded when you stand up.
Signs of dehydration
Mild dehydration can show up as thirst, dark yellow urine, dry mouth, tiredness, headaches or dizziness. In older adults, thirst signals can be weaker, so it’s worth building regular drinking habits rather than waiting to feel thirsty. Severe dehydration — confusion, very little urine, rapid heartbeat — needs urgent medical attention.
Does coffee count towards your water intake?
Yes. Although caffeine has a mild diuretic effect, research suggests that moderate amounts of coffee and tea still contribute to your overall hydration. The same goes for milk, juice and soup. Water is still the best everyday choice because it has no sugar or calories, but you don’t need to cancel out your morning coffee with an extra glass.
Can you drink too much water?
It’s rare, but yes. Drinking very large amounts in a short time can dilute the sodium in your blood, a condition called hyponatraemia. It’s most often seen in endurance athletes who drink far beyond what they sweat out. For everyday life, drinking to thirst prevents it.
Easy ways to drink more water
- Keep a refillable bottle on your desk or in your bag.
- Drink a glass of water with every meal.
- Start the day with a glass of water — it fits neatly into a morning routine.
- Flavour water with lemon, cucumber, mint or berries.
- Eat more water-rich foods: cucumber, watermelon, oranges, lettuce and soups.
- Use reminders on your phone or watch if you tend to forget.
Water and exercise
If you’re active, drink before, during and after exercise. For workouts under an hour, water is enough. For long or intense sessions in the heat, a drink with electrolytes can help replace the sodium you sweat out. A simple check: weigh yourself before and after a long workout — each kilogram lost is roughly a litre of fluid to replace. If you’re just getting started, our guide on starting exercise when you’re out of shape covers building up gently.
Does drinking water help you sleep or lose weight?
Staying hydrated supports energy and concentration, and swapping sugary drinks for water is one of the simplest ways to cut calories. Drinking a glass before meals may help some people eat a little less. For sleep, it’s the timing that matters: drink well during the day and taper off in the evening to avoid waking up at night. Our guide on improving sleep quality has more on evening habits.
Frequently asked questions
How much water should you drink a day?
As a guide, about 2.7 litres of total fluid a day for women and 3.7 litres for men, including water from food and other drinks. Needs vary with activity, climate and health.
Is 2 litres of water a day enough?
For many adults, around 2 litres from drinks plus water from food is enough, especially in a mild climate with light activity.
What colour should urine be if I’m hydrated?
Pale yellow, similar to light straw. Dark yellow suggests you need more fluid.
Does tea count as water intake?
Yes. Tea, coffee, milk and other drinks all count toward your daily fluid intake.
