Key facts
- A common guideline is 35 ml of water per kg of body weight per day.
- Exercise, heat and illness all raise your needs.
- Food provides roughly 20% of daily water — the rest comes from drinks.
- Pale-yellow urine is a good sign you are drinking enough.
How much water should you drink daily? A widely used rule is 35 ml per kilogram of body weight, adjusted for activity. This free water intake calculator turns your weight and activity level into a personal daily target in litres and glasses.
Enter your weight and pick your activity level. Your recommended intake appears instantly, with the maths shown so you can see exactly where the number comes from.
How to use
- Enter your weight in kilograms.
- Choose your activity level.
- Read your daily target in litres, millilitres and glasses.
Water intake formula
| Term | Meaning |
|---|---|
| weight × 35 | Base intake: 35 ml per kg of body weight. |
| Activity bonus | +0 ml sedentary, +500 ml moderate, +1,000 ml very active. |
Worked examples
Desk worker
70 kg, sedentary: 70 × 35 = 2,450 ml — about 2.45 litres or 10 glasses.
Active lifestyle
80 kg, very active: 80 × 35 + 1,000 = 3,800 ml — about 3.8 litres or 15 glasses.
Daily targets by weight (sedentary)
| Weight | Daily water | Glasses (250 ml) |
|---|---|---|
| 50 kg | 1.75 L | 7 |
| 60 kg | 2.10 L | 8–9 |
| 70 kg | 2.45 L | 10 |
| 80 kg | 2.80 L | 11 |
| 90 kg | 3.15 L | 12–13 |
| 100 kg | 3.50 L | 14 |
Frequently asked questions
Is the "8 glasses a day" rule correct?
It is a rough average that happens to suit a 60–70 kg sedentary adult. Heavier or active people need more — that is what this calculator adjusts for.
Does tea or coffee count?
Mostly yes. Caffeine is mildly diuretic, but the water in tea and coffee still hydrates you. Plain water remains the best default.
Can you drink too much water?
Yes, though it is rare. Extremely high intake in a short time can dilute blood sodium (hyponatremia). Spread your target across the day instead of chugging it at once.
Do I need more water in summer?
Yes. Heat and sweating raise fluid loss noticeably — add an extra 500 ml to 1 litre on hot days or when exercising outdoors.
What are signs of dehydration?
Dark urine, headache, tiredness, dry mouth and dizziness. Thirst alone is a late signal — drink on schedule, not just when thirsty.
Key takeaways
- Start from 35 ml per kg of body weight, then add for activity and heat.
- A 70 kg desk worker needs about 2.5 litres — roughly 10 glasses.
- Spread intake through the day; pale-yellow urine means you are on track.
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