Water Intake Calculator
Work out roughly how much water to drink each day from your weight and activity level — shown in litres and glasses — alongside the NHS 6–8 glasses guide.
Daily water target
By weight and activity. NHS 6–8 glasses guide.
A general guide only. The NHS recommends 6–8 glasses (about 1.5–2 litres) of fluid a day — see NHS — water and drinks.
How much water should you drink a day?
There's no single right answer, but a useful estimate is about 33 ml of water per kilogram of body weight, adjusted for exercise and hot weather. The NHS frames it more simply: aim for 6 to 8 glasses (roughly 1.5–2 litres) of fluid a day. This calculator personalises that range from your weight and activity so you have a clear daily target.
At about 33 ml per kg, a 70 kg adult needs roughly 2.3 litres of water a day before exercise — close to the NHS 1.5–2 litre guide once you count fluid from food and other drinks. Add about 350 ml for every 30 minutes of exercise.
What counts towards your fluid
| Source | Counts? |
|---|---|
| Water, milk, sugar-free drinks | Yes |
| Tea and coffee (in moderation) | Yes |
| Fruit and vegetables (~20% of fluid) | Yes |
| Alcohol | No (it's dehydrating) |
Source: NHS — water, drinks and your health.
Signs you need more
Dark yellow urine, feeling thirsty, tired or lightheaded can all signal you need to drink more. In hot weather, when exercising, or if you're unwell with a fever, vomiting or diarrhoea, your needs rise. You can have too much water in rare cases, so use this as a guide, not a strict rule — and follow medical advice if you have a kidney or heart condition.
Frequently asked questions
How much water should I drink a day?
The NHS suggests 6 to 8 glasses, or about 1.5 to 2 litres of fluid a day. By body weight, roughly 33 ml per kilogram is a common estimate, so a 70 kg adult needs around 2.3 litres before exercise.
Does tea and coffee count towards my water intake?
Yes. The NHS confirms tea, coffee, milk and sugar-free drinks all count towards your daily fluid, and food provides around a fifth of it. Only alcohol doesn't count, as it has a dehydrating effect.
How much extra water do I need when exercising?
A rough guide is an extra 350–500 ml for every 30 minutes of exercise, more in hot conditions. Drink before, during and after activity, and listen to your thirst.
Can I drink too much water?
Very rarely, drinking excessive amounts in a short time can dilute blood sodium dangerously. For almost everyone the risk is drinking too little, but people with certain heart or kidney conditions should follow their doctor's advice.
How do I know if I'm dehydrated?
Pale straw-coloured urine usually means you're well hydrated. Dark urine, thirst, tiredness, a dry mouth or feeling lightheaded are signs to drink more fluid.