How much water you actually need
A common rule of thumb is about 30–35 ml of water per kilogram of body weight per day, which this calculator uses as its base, then adds for exercise and hot weather. For a 70 kg adult that is roughly 2.3 litres before training. Health authorities often quote totals around 2.7 litres for women and 3.7 litres for men, but crucially that figure includes water from food and all drinks, not just plain water. Your needs rise with sweat, so the more you move and the hotter it is, the more you should drink.
Exercise and climate
Exercise is the biggest variable. You lose water as sweat to cool down, and hard or long sessions can cost a litre or more an hour. This calculator adds about 350 ml for every 30 minutes of exercise as a rough replacement, and increases the total in hot or humid conditions where you sweat more. On heavy training days, drink to thirst around your sessions and consider electrolytes for efforts over about an hour, since you lose salts as well as water. Altitude and air travel also increase needs.
Reading your own signals
Your body is a good guide. Pale, straw-coloured urine usually means you are well hydrated; dark yellow suggests you need more. Thirst, headaches, fatigue and poor concentration can all signal mild dehydration. You do not need to force water far beyond thirst, and drinking very large amounts quickly can be harmful by diluting blood sodium. Spread intake through the day rather than gulping it all at once, and drink a little more when the calculator's assumptions — heat, exercise — apply to you.
Making hydration easy
Keep a bottle visible, drink a glass with each meal, and top up around exercise. Foods like fruit, vegetables and soup contribute meaningfully to your total. Treat this calculator's number as a target to aim near, not a strict quota, and adjust for how you feel and what you are doing. These tools use standard, published formulas and the figures you enter. They are estimates for general fitness and planning, not a medical assessment — check with a doctor or dietitian before making health decisions.
Frequently asked questions
How much water should I drink a day?
Roughly 30–35 ml per kilogram of body weight, plus more for exercise and heat. For many adults that lands between 2 and 3 litres of total fluid, including food and other drinks.
Does coffee and tea count toward water intake?
Yes. Despite their caffeine, tea, coffee and most drinks contribute to hydration, and water-rich foods add more. The total includes all of these, not just plain water.
How much extra water do I need when exercising?
About 350–700 ml per 30–60 minutes of exercise as a rough guide, more in heat. Drink to thirst around sessions and use electrolytes for efforts over an hour.
Can I drink too much water?
Yes, though it is uncommon. Drinking very large volumes quickly can dangerously dilute blood sodium. Spread intake through the day and drink mainly to thirst plus training needs.