Water Intake Calculator – Recommended Daily Water

Daily water intake: guidelines and science

How much water you need each day depends on your body size, activity, and environment. Most guidelines use a simple rule of thumb: 30–40 ml per kilogram of body weight per day. That range comes from population studies and clinical practice. It is easy to remember and works reasonably well for most healthy adults when adjusted for how active they are and whether they live or work in a hot or humid climate.

Why 30–40 ml per kg?

The 30–40 ml/kg range reflects average fluid needs for maintenance—enough to replace what we lose through urine, sweat, breathing, and stool. Smaller or sedentary people tend to sit toward the lower end; larger or more active people toward the upper end. It is not derived from a single study but from decades of nutrition and sports science. Many health authorities avoid a single "magic number" and instead recommend listening to thirst while using such guidelines as a starting point.

Thirst vs. guideline

Thirst is your body's way of saying it needs more fluid. For most people, drinking when thirsty is a good strategy. Guidelines (like this calculator) are useful when you want a target number—for example, to build a habit or to plan intake on busy or active days. Some people (e.g. older adults or athletes) may not feel thirst strongly enough, so having a rough target can help avoid mild dehydration.

Signs of dehydration

Mild dehydration can show up as dry mouth, darker urine, tiredness, or headache. More severe dehydration may cause dizziness, very little urine, or confusion. If you are often thirsty, have dark urine, or feel foggy, try increasing fluids gradually and see if you feel better. Persistent or severe symptoms warrant a doctor's visit.

Overhydration risks

Drinking far more than your body needs can, in rare cases, lead to overhydration (hyponatremia)—dilution of blood sodium. This is uncommon in everyday life and is more often seen in endurance athletes who overdrink during long events. Staying within a reasonable range (e.g. the amount this calculator suggests) and drinking according to thirst during normal days is generally safe.

Does coffee count?

Yes, in moderation. Coffee and tea contribute to total fluid intake. Caffeine has a mild diuretic effect, but in regular drinkers it does not cause a net loss of fluid. So a couple of cups of coffee can "count" toward your daily fluids. Don't rely on caffeine alone—plain water is still the best base for hydration.

Does climate matter?

Yes. In hot or humid conditions you lose more fluid through sweat, so your needs go up. Many guidelines add roughly 250–500 ml (or more) per day when you are regularly in a hot or humid environment. This calculator adds 500 ml when you select "Hot / humid" to reflect that.

Does exercise increase needs?

Yes. The more you sweat, the more you need to replace. A common recommendation is to add about 400–600 ml per hour of exercise, depending on intensity. This tool uses a simple +500 ml per hour of exercise so you can bump your daily target on days you train. Drink before, during (for long sessions), and after exercise rather than chugging a huge amount at once.