The NHS suggests about 6 to 8 glasses of fluid a day, which is roughly 1.5 to 2 litres. That is a general guide: how much you actually need depends on your size, how active you are, the weather and your health.
Water is not the only source. Tea, coffee, milk, juice and the water in food all count towards your total, though plain water is a good default because it has no sugar or calories.
The 6 to 8 glasses guide
“Eight glasses a day” is a useful rule of thumb, not a scientific law. A glass here is about 200 to 250 ml. Healthy people usually regulate fluid well through thirst, and the colour of your urine is a handy check: pale straw suggests you are well hydrated, while darker yellow suggests you could drink more.
If you have a kidney, heart or other medical condition, or are pregnant, your doctor or midwife may give different advice, so follow that.
What changes how much you need
Several things push your needs up or down:
- Body size: larger bodies generally need more fluid.
- Exercise: you lose water through sweat, so a run or gym session adds to your needs.
- Hot or humid weather: more sweating means more to replace.
- Illness: fever, vomiting or diarrhoea can dehydrate you quickly.
- Pregnancy and breastfeeding: needs differ, so check with your midwife or GP.
- Food: about a fifth of the water you take in typically comes from food, particularly fruit and vegetables.
Our Water intake tool combines your weight, exercise time and climate into one estimate, then shows it in litres, glasses and fluid ounces.
A worked example
Take someone who weighs 70 kg, exercises for 30 minutes a day and lives in a mild climate. The calculator gives:
| Component | Amount |
|---|---|
| Baseline for body weight | 2,310 ml |
| Extra for exercise | 350 ml |
| Extra for hot weather | 0 ml |
| Total | about 2.7 litres |
That works out at around 11 glasses of 250 ml, or about 90 US fluid ounces. Because a fifth or so of your water comes from food, your drinks can be a little lower than that total.
This is higher than the NHS 6 to 8 glasses guide. The two are not in conflict. The NHS figure is a general minimum for ordinary days, while the calculator scales up for a heavier body and for exercise.
Signs you may not be drinking enough
Mild dehydration can show up as thirst, a dry mouth, tiredness, a headache or difficulty concentrating. Urine that is dark and strong-smelling is another clue. These signs are not specific, so they can have other causes, but if they ease after a drink, fluid was probably part of it.
Older adults can feel thirst less strongly, and babies and young children need careful attention in hot weather or illness. If you are caring for someone vulnerable, offer drinks regularly rather than waiting to be asked.
Tips for drinking enough
- Keep a bottle or glass in view, at your desk or in your bag.
- Drink a glass with each meal and one when you wake up.
- Add a slice of lemon or a splash of squash if plain water bores you.
- Drink to thirst, and a little more on hot days or after exercise.
- If you are training for an event, read Running pace and race finish times explained to plan your efforts, and take fluids to suit the conditions.
Hydration also ties into energy and appetite. Our guide to Calories and TDEE: how many calories do you need? explains how much food your body needs, and Sleep cycles: how to choose a bedtime or wake-up time covers rest, which affects thirst and recovery too.
When to be careful
You can drink too much. Very large amounts in a short time can dilute the salts in your blood, which is rare but serious. Spread your drinking through the day rather than forcing down large volumes at once.
If you are constantly very thirsty, pass large amounts of urine, or feel faint or confused, speak to a GP or call NHS 111.
Note: This is general information, not medical advice. Our calculator gives an estimate only. If you have a medical condition, follow your doctor’s guidance on fluids.
Try the calculator
Use the Water intake calculator to estimate your daily intake from your weight, exercise and the weather where you live.
Worked example 2: how weight, exercise and heat add up
The calculator uses a baseline of about 33 ml of water per kilogram of body weight, adds 350 ml for each 30 minutes of daily exercise and adds 500 ml for hot or humid weather. Here is how the total changes for four different people:
| Person | Baseline | Exercise | Hot weather | Total | Glasses of 250 ml |
|---|---|---|---|---|---|
| 55 kg, no exercise, mild | 1,815 ml | 0 | 0 | 1.8 litres | 7 |
| 60 kg, no exercise, mild | 1,980 ml | 0 | 0 | 2.0 litres | 8 |
| 85 kg, 60 min exercise, mild | 2,805 ml | 700 ml | 0 | 3.5 litres | 14 |
| 85 kg, 60 min exercise, hot | 2,805 ml | 700 ml | 500 ml | 4.0 litres | 16 |
The figures are House of Calculator estimates. They are a guide to total water needs, and some of that comes from food and from other drinks such as tea and milk. Someone in the last row would not need to drink all four litres as plain water, and should listen to thirst, but it shows how quickly needs change between a quiet day and a hot day with training.
Drinks and foods that count
Almost everything that contains water contributes. Roughly, in order of how helpful they are as everyday choices:
- Water and sparkling water: no sugar and no calories.
- Milk and unsweetened plant drinks: provide fluid plus nutrients.
- Tea and coffee: count towards fluid intake, but caffeine can make you want the toilet more, and too much can affect sleep. See Sleep cycles: how to choose a bedtime or wake-up time for how late caffeine affects rest.
- Fruit juice and smoothies: count, but the NHS advises limiting them because of sugar, to a small glass a day.
- Sugary soft drinks and energy drinks: count towards fluid but add sugar, so they are not a good main source.
- Alcohol: does not hydrate you well. If you drink, alternate with water, and see Alcohol units explained: how to count units in your drinks for how units are counted.
- Food: soups, fruit, vegetables, yoghurt and salads all contain a lot of water.
Hydration for different situations
Exercise. For a short, easy session, water is enough. For longer or hotter efforts, drink little and often rather than a lot at once, and consider whether you need salts. The pace and effort of your session affect how much you sweat. Our guide to Heart rate zones explained: how to find your maximum and train by zone describes how hard different efforts are.
Hot weather. Needs rise as you sweat more, so keep drinks within reach, avoid strenuous activity in the hottest part of the day, and watch for signs of heat exhaustion such as headache, dizziness and heavy sweating.
Illness. Fever, vomiting and diarrhoea raise your fluid needs. Take small, frequent sips. If you cannot keep fluids down, or an infant or older adult is unwell, get medical advice.
Pregnancy and breastfeeding. Your needs may be higher. Follow the guidance from your midwife or GP.
Older age. Thirst can be weaker, so drink regularly through the day.
Work and travel. Air conditioning, long flights and busy days can lead you to forget drinks. A refillable bottle is a simple prompt.
Common myths about water
- “You must drink eight glasses of plain water.” The NHS guidance is 6 to 8 glasses of fluid, and other drinks count.
- “If you feel thirsty you are already seriously dehydrated.” Thirst is the body’s early signal. For most healthy people it is a reliable guide.
- “Clear urine is the goal.” Pale straw is the usual target. Completely clear urine on most occasions may mean you are drinking more than you need.
- “Coffee dehydrates you.” In usual amounts, coffee and tea still add to your fluid intake.
- “More is always better.” Drinking far more than you need does not give extra benefit and in rare cases can be harmful.
Step-by-step: building a simple daily routine
- Start with the calculator and note the estimate in litres.
- Subtract about a fifth for water from food, if you want a rough figure for drinks.
- Split the target over the day: a glass when you wake up, one with each meal, one mid-morning and mid-afternoon, and some around exercise.
- Use your urine colour as a check and adjust.
- Review on hot, busy or active days and when you are ill.
Hydration is only one part of how your body is fuelled; Calories and TDEE: how many calories do you need? covers energy needs.
When to check with a doctor or the NHS
Some medical conditions, including kidney, heart and liver disease, and some medicines, change how much fluid is safe, so follow your care team’s advice. Seek help from a GP or NHS 111 if you have persistent extreme thirst, pass unusually large amounts of urine, notice very dark urine that does not improve with drinking, or feel faint, confused or very drowsy. In an emergency, call 999.
Frequently asked questions
Does tea or coffee count towards my water intake?
Yes. Tea, coffee, milk and juice all contribute to your fluid intake. Water, milk and unsweetened drinks are generally the best choices.
How do I know if I am drinking enough?
Pale straw-coloured urine and rarely feeling thirsty are good signs. Dark yellow urine, a dry mouth or a headache can suggest you need more.
Do I need more water when I exercise?
Usually, yes. You lose fluid through sweat, so top up before, during and after longer or hotter sessions.
Is the eight glasses a day rule true?
It is a handy guide rather than a precise rule. The NHS gives 6 to 8 glasses, and your own needs vary.
Can I drink too much water?
Yes, although it is uncommon. Drinking very large amounts in a short time can dilute the salts in your blood. Spread fluids through the day and drink to thirst.
Does sparkling water hydrate as well as still water?
Yes, plain sparkling water contributes to your fluid intake in the same way. Flavoured versions may contain sugar or acids, so check the label.
How much water should children drink?
It depends on age and size, and guidance differs. The NHS publishes recommendations by age, so check its advice for your child and offer drinks regularly, especially in hot weather.
Should I drink water before bed?
A glass is fine for most people, but a large amount may lead to waking in the night. Drink more earlier in the day if that is a problem.