How much water should I drink a day?
There is no single number that suits everyone. How much you need depends on your body size, how active you are, the weather, what you eat and your health. Official guidance gives a useful starting point:
- UK (NHS): aim for 6 to 8 cups or glasses of fluid a day, as a guide. Water, lower-fat milk and lower-sugar drinks, including tea and coffee, all count.
- Europe (EFSA, 2010): about 2.0 liters of total water a day for women and 2.5 liters for men, counting drinks and the water in food, for moderate temperatures and moderate activity.
- US (National Academies, 2004): about 2.7 liters (91 fl oz) of total water a day for women and 3.7 liters (125 fl oz) for men, from all drinks and food. Around 80% of it usually comes from drinks.
These are averages for healthy adults, not targets you must hit. For most people, drinking when thirsty and with meals is enough. People who are very active or spend time in the heat may need more.
How this water intake calculator works
The calculator starts from your body weight and adds a little extra for exercise and warm weather. It is a rule of thumb based on common guidance, not a measurement of what your body needs.
| Step | Metric | US units |
|---|---|---|
| Body weight | 33 ml per kg | ½ fl oz per lb |
| Lightly active (about 30 min of exercise a day) | +350 ml | +12 fl oz |
| Moderately active (about 60 min) | +700 ml | +24 fl oz |
| Very active (90 min or more) | +1,050 ml | +36 fl oz |
| Warm climate | +250 ml | +8 fl oz |
| Hot or humid climate | +500 ml | +17 fl oz |
The total is rounded to the nearest 100 ml and shown in liters, or in fluid ounces and 8 fl oz cups if you choose US units. The body-weight step is the familiar "half your body weight in ounces" rule, and the activity step adds about 350 ml (12 fl oz) for every 30 minutes of exercise.
Example: someone who weighs 70 kg (154 lb), walks for about 30 minutes a day and lives in a mild climate gets 70 × 33 ml = 2,310 ml, plus 350 ml for activity. That is about 2.7 liters (91 fl oz) a day, or roughly 11 glasses of 250 ml.
The tracker uses this estimate as your daily goal. Until you enter a weight it uses 2 liters, and you can set your own goal in the settings if you prefer a different number.
Tips for drinking more water at your desk
- Keep a glass or refillable bottle on your desk, where you can see it.
- Have a few sips with every break. If you use the 20-20-20 eye-break timer, its breaks are an easy moment to drink.
- Drink a glass with each meal, and alongside your morning tea or coffee.
- Turn on sip reminders above. They only run during your active hours.
- If plain water feels boring, add a slice of lemon or cucumber or a few mint leaves, or try it sparkling.
- Refill your glass whenever you get up, so the next drink is already waiting. The stand-up timer reminds you to get up every 45 minutes.
Signs you may need to drink more
The color of your pee is a simple guide: the NHS suggests drinking enough during the day for it to be a clear, pale yellow. Common signs that you may not be drinking enough include:
- feeling thirsty
- dark yellow, strong-smelling pee
- peeing less often than usual
- feeling dizzy, light-headed or tired
- a dry mouth, lips and tongue
If you notice these, have a drink. If you still feel unwell, or the symptoms don't improve, speak to a pharmacist or doctor.
When to follow a doctor's advice instead
This calculator is meant for healthy adults. Your needs may be quite different, and advice from your doctor, midwife or dietitian should come first, if you:
- are pregnant or breastfeeding
- have a kidney, heart or liver condition
- have been told to limit how much you drink
- take medicines that affect your fluid balance, such as diuretics
- are older, as the sense of thirst can become weaker with age
- are working out how much a child should drink
Supabreak provides general wellbeing reminders and information, not medical advice.