What is hydration?
Hydration means having enough fluid in your body for normal functions such as temperature control, blood flow, digestion and concentration.
Hydration is the state of having enough fluid in the body for everyday functions. Fluid helps carry nutrients, regulate temperature, remove waste and keep the body working normally.
Hydration is not only about plain water. NHS guidance says water, lower-fat milk and sugar-free drinks, including tea and coffee, can count towards daily fluid intake. Some fluid also comes from food.
Why hydration matters
Good hydration supports normal body functions such as temperature control, concentration, digestion, joint lubrication and the movement of nutrients and waste products around the body.
Hydration also sits alongside wider nutrition. Fluids are not a replacement for balanced meals, enough energy, fibre, fruit and vegetables, protein, carbohydrates and fats.
Estimate your daily fluid needs
Use the water intake calculator for a general daily fluid estimate based on body weight, activity and weather.
How much fluid should adults drink?
NHS guidance says most people should aim for 6 to 8 cups or glasses of fluid a day. This is a general guide, not a personalised target for every person.
What counts towards fluid intake?
Plain water is a simple choice, but it is not the only fluid that counts. NHS guidance says lower-fat milk and sugar-free drinks, including tea and coffee, can also contribute.
| Drink or source | Counts? | Useful note |
|---|---|---|
| Water | Yes | A simple low-cost option with no sugar or calories. |
| Lower-fat milk | Yes | Can provide fluid plus nutrients such as calcium. |
| Tea and coffee | Yes | Can count towards fluid intake, although caffeine sensitivity varies. |
| Sugar-free drinks | Yes | Can contribute fluid without added sugar. |
| Fruit juice and smoothies | Limited | NHS guidance recommends limiting these to 150ml a day. |
| Alcohol | No reliable hydration source | Alcohol can increase urination and should not be used as a hydration strategy. |
Worked example
Suppose someone uses a broad calculator estimate of 33ml per kilogram of body weight and weighs 70kg.
70kg × 33ml = 2,310ml per day
2,310ml ÷ 250ml glass = about 9 glassesThis is only a planning estimate. It should be compared with thirst, urine colour, activity, weather and medical advice. It is not better to force excessive fluid beyond what is sensible for your circumstances.
When fluid needs may change
Fluid needs are not fixed. NHS guidance says people may need more fluid if they are pregnant or breastfeeding, in a hot environment, physically active for long periods, ill or recovering from illness.
- Exercise: longer or sweatier activity can increase fluid needs.
- Hot weather: higher temperatures can increase sweating and water loss.
- Pregnancy or breastfeeding: needs can change, so midwife or clinician guidance matters.
- Illness: fever, vomiting or diarrhoea can affect hydration and may need medical advice.
- Medical fluid restrictions: kidney, heart or other conditions may require a personalised fluid limit.
Signs you may not be drinking enough
One practical NHS-style cue is urine colour: many people should aim for pee that is a clear pale yellow. Darker urine can be a sign you may need more fluid, although supplements, medicines and some foods can also affect colour.
Hydration FAQs
Does hydration mean drinking only water?
No. Water is a good choice, but NHS guidance says lower-fat milk and sugar-free drinks, including tea and coffee, can also count towards fluid intake.
Is 8 glasses of water a day always enough?
Not always. Six to eight glasses is a general guide. Some people may need more or less depending on activity, weather, pregnancy, breastfeeding, illness or medical advice.
Can you drink too much water?
Yes, excessive fluid intake can be unsafe, especially in endurance exercise or certain medical conditions. Do not force extreme water intake.
What is the easiest sign to check?
For many people, urine that is clear pale yellow is a useful everyday sign. Very dark urine can suggest you may need more fluid, but symptoms and medical context matter too.
Sources and notes
- NHS hydration guidance: NHS — Water, drinks and hydration
- British Dietetic Association fluid guidance: BDA — Fluid, water and drinks
- British Nutrition Foundation hydration information: BNF — hydration