Water Intake Calculator

How much water do you need a day?

There is no single magic number that fits everyone, which is why old rules like eight glasses a day are only rough guides. A more personal baseline is 30 to 40 millilitres of fluid per kilogram of body weight, which for many adults works out between 2 and 3 litres a day. On top of that, exercise and hot weather increase your needs as you lose fluid through sweat.

How this calculator works

We start from your body weight to set the 30 to 40 ml per kilogram range, then add about 350 ml for every 30 minutes of exercise you log, and a further allowance in hot or humid conditions where sweat losses climb. The result is a range rather than a single figure, because appetite, activity, health, and even the food you eat all move the target around. Roughly a fifth of most people's fluid comes from food, especially fruit, vegetables, and soups, so you do not need to drink the entire amount as plain water. Baseline sources are on our methodology page.

How do you know if you are well hydrated?

Pale yellow urine and only occasional thirst are good everyday signs. Dark urine, headaches, or unusual fatigue can mean you need more. For most healthy people, drinking to thirst plus this baseline is plenty, and it is better to spread intake through the day than to force large volumes at once. If you follow a low carbohydrate plan, fluid and salt needs rise in the early weeks, which the keto calculator explains, and staying hydrated helps on longer sessions logged in the calories burned walking calculator.

When should you ignore this calculator?

This is general guidance, not a medical prescription. Kidney disease, heart failure, some medications, and pregnancy all change fluid needs, so follow your doctor's advice if any of those apply. Endurance athletes sweating heavily for hours also need electrolyte planning beyond a simple volume target. For everyday life, pair your fluid target with the calorie side of the plan from the TDEE calculator.

How much water should I drink for my weight?

Body weight is the most useful starting point, which is why this works as a water intake calculator by weight rather than a flat eight-glasses rule. Heavier bodies hold and lose more fluid, so the 30 to 40 ml per kilogram range scales with you: a 60 kg adult lands near 1.8 to 2.4 litres before exercise, while a 90 kg adult is closer to 2.7 to 3.6. The National Academies note a total water intake of roughly 2.7 litres a day for women and 3.7 for men from all food and drink combined, which lines up with the weight-based range for most people once food fluid is included.

Limitations of this calculator

This is a general estimate of total fluid, not a medical target. Real needs shift with climate, altitude, illness, breastfeeding, and how much water-rich food you eat, and some conditions such as kidney or heart disease require a doctor to set fluid limits. Drinking far more than you need offers no benefit and, in extreme amounts taken quickly, can lower blood sodium dangerously. Treat the range as a guide, spread intake through the day, and let thirst and urine colour fine-tune it.

Factors that change your water needs

• Body weight: the baseline scales at 30 to 40 ml per kilogram, so larger bodies need more.
• Exercise: each 30 minutes adds roughly 350 ml to replace sweat, more for long or intense sessions.
• Climate: heat, humidity, and altitude raise fluid losses considerably.
• Diet: water-rich fruits and vegetables supply fluid, while low-carbohydrate plans increase early water and sodium needs.
• Health and medication: fever, illness, pregnancy, breastfeeding, and some medicines change requirements.
• Caffeine and alcohol: normal coffee and tea count toward fluid; heavy alcohol is dehydrating.