Water Intake Calculator: How Much Water Should You Drink?
Water Intake Guide
A water intake calculator estimates how much fluid your body needs each day, using your weight as the main input. It replaces vague rules of thumb with a number that scales to your size — a helpful baseline you can then adjust for activity, climate, and how you feel.
The weight-based estimate
A common approach uses roughly 30 to 35 millilitres of water per kilogram of body weight per day. Multiply your weight by a value in that range to get a daily target in millilitres, then divide by 1,000 for litres.
daily water (ml) = weight(kg) × 30 to 35
A worked example
For a 70 kg person, using 33 ml/kg:
70 × 33 = 2,310 ml ≈ 2.3 litres per day
That total includes water from all drinks and, to some extent, food. It is a starting point, not a strict quota.
What about the "8 glasses" rule?
The familiar "eight glasses a day" advice lands near two litres, so it is a reasonable rough guide — but it ignores body size and lifestyle. A large, active person in a hot climate needs considerably more than a smaller, sedentary one. That is why a weight-based estimate is usually more useful.
Factors that raise your needs
- Exercise, especially long or intense sessions where you sweat.
- Hot or humid weather, and high altitude.
- Illness with fever, vomiting, or diarrhoea.
- Pregnancy and breastfeeding.
How to use the calculator online
- Enter your body weight.
- Optionally set an activity level.
- Read your estimated daily water target.
- Adjust upward on hot or active days.
This is general information, not medical advice. A useful everyday check is urine colour: pale yellow usually signals good hydration.
FAQ
Q: Is eight glasses a day correct?
A: It is a rough guide that happens to land near two litres. A weight-based estimate is more personalized because it scales with your size.
Q: Do coffee and tea count?
A: Mostly yes. Despite a mild diuretic effect, the fluid in everyday coffee and tea still contributes to your daily total.
Q: How much extra do I need when exercising?
A: Add fluid to replace what you sweat out — often several hundred millilitres per hour of activity, more in heat.