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.
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.