Complete guide
Reviewed July 2026Water runs everything in the body - temperature control, digestion, circulation, joint lubrication, waste removal and the delivery of nutrients to every cell. Yet 'how much should I drink?' gets answered with a one-size-fits-all '8 glasses a day' that has no real scientific basis. Your true need depends on your body size, how much you move, the climate you live in, and what else you eat and drink.
This calculator estimates your daily water target from body weight and activity, giving a figure in litres and in glasses. Below you'll find the formula, worked examples, the factors that raise or lower your needs, the myths worth dropping, and the honest signs that tell you whether you're actually hydrated.
The best guide isn't a fixed number at all - it's your body. This calculator gives a sensible target; your thirst and urine colour confirm whether you're hitting it.
How much water do you need?
Baseline = weight(kg) x 30-35 ml Exercise addition = about 350-500 ml per 30 min of activity Daily target = baseline + exercise addition (+ extra for heat)
A widely used rule of thumb is 30-35 ml of water per kilogram of body weight for baseline needs, then more for exercise and hot weather. This scales with body size - a 90 kg person genuinely needs more than a 50 kg person, which the '8 glasses' rule ignores entirely.
Worked examples
- 70 kg, lightly active: baseline = 70 x 33 ml = 2,310 ml, roughly 2.3 litres or about 9 glasses of 250 ml.
- 90 kg, 1 hour of exercise: baseline 90 x 33 = 2,970 ml, plus about 700-900 ml for the workout = roughly 3.7-3.9 litres.
- 55 kg, sedentary, cool climate: 55 x 30 = 1,650 ml, about 1.65 litres or ~7 glasses.
- Add for heat/humidity: hot-weather sweating can add 0.5-1 litre or more; listen to thirst and check urine colour.
What changes your needs (and what doesn't)
Factors that raise your needs
- Exercise and heavy sweating - the biggest variable.
- Hot or humid climate, and high altitude.
- Pregnancy and breastfeeding.
- Fever, vomiting or diarrhoea (fluid loss).
- High-protein or high-fibre diets, which use more water to process.
- Caffeine and alcohol, which are mildly diuretic (though moderate coffee still counts toward fluids).
Myths worth dropping
| Myth | Reality |
|---|---|
| Everyone needs 8 glasses | Needs scale with weight, activity and climate |
| Only plain water counts | Food (~20% of intake), tea, coffee, milk all count |
| Coffee dehydrates you | Moderate coffee is a net positive for hydration |
| Thirst means you're already dehydrated | Thirst is a normal, timely signal - not an emergency |
| More water is always better | Excess can cause hyponatremia (low blood sodium) |
Using this calculator
- Enter your body weight and typical daily activity.
- Read the target in litres and glasses as a baseline.
- Add roughly 0.5-1 litre on hot days or heavy-exercise days.
- Confirm with your body: aim for pale-yellow urine and drink to thirst rather than forcing a fixed quota.
Common mistakes
- Blindly following '8 glasses' regardless of your size or activity.
- Counting only plain water and ignoring food, tea, coffee and milk (about 20% of intake comes from food).
- Front-loading or chugging large amounts at once instead of spreading intake through the day.
- Ignoring extra needs during exercise, heat, illness or pregnancy.
- Over-drinking to the point of very frequent clear urination - excessive water can dilute blood sodium dangerously.
Frequently asked questions
Glossary
- Hydration
- Maintaining adequate body water for normal function.
- Baseline needs
- Water required for basic function, scaled to body weight.
- Diuretic
- A substance that increases urine output, like caffeine or alcohol.
- Hyponatremia
- Dangerously low blood sodium, possible from overhydration.
- Electrolytes
- Minerals like sodium and potassium lost in sweat and needed for balance.
- Fluid from food
- The ~20% of daily water obtained from what you eat.
- Urine colour test
- Using urine shade as a practical hydration indicator.
- Sweat rate
- How much fluid you lose during exercise, a key personal variable.
Key takeaways
Daily water needs scale with body weight (about 30-35 ml/kg), plus more for exercise, heat, pregnancy and illness - the '8 glasses' rule ignores all of this. Food supplies roughly 20% of intake, and moderate coffee and tea count too. Use a weight-based target as a guide, but let your body decide: pale-yellow urine and normal thirst mean you're on track, while both too little and too much water carry risks.
Enter your weight and activity above for a personalised hydration target; then confirm it against the simplest test there is - your urine colour and your thirst.