Most healthy adults do well with roughly 1.5 to 2 litres of drinks a day (about 6 to 8 glasses), which is what the UK's NHS suggests. Your total water needs are higher than that, around 2 litres for women and 2.5 litres for men according to the European Food Safety Authority, but part of it comes from food. Exercise, heat, illness, pregnancy and breastfeeding all push the number up.

So the famous "8 glasses a day" is a decent starting point, not a law. The rest of this guide explains where the numbers come from, how to adjust them for your body and your day, and how to tell whether you are actually getting enough.

Where the official numbers come from

Health agencies don't publish a single magic number, because water needs vary a lot from person to person. What they publish are "adequate intakes": amounts that cover the needs of most healthy people.

  • European Food Safety Authority (EFSA): about 2.0 litres of total water a day for women and 2.5 litres for men.
  • US National Academies of Sciences: about 2.7 litres a day for women and 3.7 litres for men, again as total water.
  • NHS (UK): 6 to 8 cups or glasses of fluid a day as a practical everyday guide.

The key phrase is total water. That includes the water in food, which typically provides around a fifth of your intake. Soups, fruit, vegetables, yogurt, rice and pasta all contain plenty of it. That's why the "drinks" target (6 to 8 glasses) is lower than the "total water" figures.

A simple way to estimate your own target

If you like a personal number, a common rule of thumb is about 30 to 35 ml of fluid per kilogram of body weight per day. It is an estimate, not a prescription, but it scales sensibly with size.

  • 55 kg (121 lb): roughly 1.7 to 1.9 litres
  • 70 kg (154 lb): roughly 2.1 to 2.5 litres
  • 90 kg (198 lb): roughly 2.7 to 3.2 litres

Then adjust for your day:

  • Exercise: add roughly 400 to 800 ml for each hour of sweaty training. Our guide to hydration before, during and after exercise goes into detail.
  • Hot or humid weather: you can sweat a lot even without exercising. Drink more and don't wait for thirst to become strong.
  • Illness: fever, vomiting or diarrhoea increase losses. Small, frequent sips and an oral rehydration solution can help; see a doctor if you can't keep fluids down.
  • Pregnancy and breastfeeding: needs go up. EFSA suggests about an extra 300 ml a day in pregnancy and around 700 ml while breastfeeding.
  • Older adults: the sense of thirst tends to weaken with age, so a routine (a glass with every meal) matters more than waiting to feel thirsty.

What counts toward your water intake

Plain water is the best default: no calories, no sugar, nothing to think about. But it's not the only thing that counts.

  • Milk and unsweetened plant drinks count and bring protein and minerals.
  • Tea and coffee count too. Caffeine has a mild diuretic effect, but in moderate amounts the drink still hydrates you overall. Keep total caffeine moderate; health agencies generally treat up to about 400 mg a day as fine for most healthy adults (less in pregnancy).
  • Sparkling water and flavoured water without sugar are fine.
  • Juice and smoothies count but are calorie-dense. Many guidelines suggest keeping them to a small glass a day.
  • Sugary soft drinks hydrate but add a lot of sugar and calories. If you're trying to lose weight, swapping them for water is one of the easiest wins; more on that in does drinking water help you lose weight.
  • Alcohol does not help. It increases urine output, so alternate alcoholic drinks with water.

Signs you're drinking enough (and when you're not)

You don't need to measure every millilitre. Your body gives you good feedback.

  1. Urine colour: pale straw or light yellow usually means you're well hydrated. Dark yellow or amber means drink more. Completely clear all day can mean you're overdoing it.
  2. Thirst: for most healthy adults, drinking when thirsty plus with meals is enough.
  3. How often you pee: a regular trip every few hours during the day is typical. Very rare, small amounts of dark urine is a sign to drink more.

Common signs of mild dehydration include thirst, dark urine, headache, tiredness, dizziness and dry mouth or lips. Even mild dehydration can make a workout feel harder than it should.

Can you drink too much water?

Yes, though it's uncommon. Drinking very large amounts in a short time can dilute the sodium in your blood, a condition called hyponatremia. It mostly affects endurance athletes who drink far beyond their sweat losses over many hours, or people forcing down litres on a challenge. Symptoms include nausea, headache, confusion and swelling.

For everyday life, the practical rule is simple: spread your drinks across the day, drink to thirst, and don't force down large volumes "just in case". The kidneys of a healthy adult can only process roughly a litre an hour, so there's no benefit in chugging.

People with heart failure, kidney disease or some other conditions are sometimes told by their doctor to limit fluids. If that's you, follow your doctor's advice rather than general targets.

Easy habits to drink enough every day

Knowing the number is the easy part. These small habits do the actual work:

  • Start the day with a glass. Keep it by the bed or the kettle so it happens automatically.
  • Pair water with things you already do: a glass with every meal, one when you sit down to work, one after each workout.
  • Keep a bottle in sight. A 500 ml or 750 ml bottle makes counting easy: three or four refills and you're there.
  • Eat water-rich food: cucumber, tomatoes, watermelon, oranges, soups and yogurt all add up.
  • Front-load your day so you're not drinking a lot right before bed and waking up at night.
  • Track it for a week or two. Logging glasses quickly shows whether you're really at 1 litre or 2. FIThim lets you log water in a tap from the Today screen and can send an optional reminder, which is handy while the habit forms.

If you're building other routines at the same time, the same principles apply as for training; see how to stay consistent with working out.

Water and the scale

Your hydration level shows up on the scale. A day of salty food or a big carbohydrate meal can add a kilo or more of water weight overnight, and good hydration often helps your body let go of that retained water. If your weight jumps around for no obvious reason, read why your weight changes from day to day before you change anything in your plan.

FAQ

Is 8 glasses of water a day a myth?

Not exactly a myth, just a simplification. Eight glasses (about 2 litres) of drinks is close to what many adults need, but it's not based on a precise calculation, and food also provides water. Treat it as a reasonable target and adjust for size, activity and climate.

Does coffee dehydrate you?

In moderate amounts, no. Coffee and tea have a mild diuretic effect, but the water in them more than makes up for it, so they still count toward your daily fluids. Very high caffeine intakes are a separate issue for sleep and heart rate.

How much water should I drink to lose weight?

There's no special "weight-loss amount". Aim for the same healthy target as above. Water helps mainly by replacing caloric drinks and, for some people, by making it easier to eat a bit less at meals.

Is it bad to drink a lot of water before bed?

It's not harmful, but it can wake you up to use the bathroom and disturb your sleep. Drink most of your fluids earlier in the day and have just a few sips in the last hour or two before bed.