Water Intake & Hydration Calculator (By Weight & Activity)
A practical daily fluid target from your weight, activity and climate — in liters, ounces and cups.
How much water should you drink a day?
The National Academies set adequate total water intake at about 3.7 liters a day for men and 2.7 liters for women — and that includes water from food (about 20%) and all beverages. So plain drinking water of roughly 2–3 liters (8–12 cups) covers most adults; more in heat or with training. There is no evidence that everyone needs “8 glasses” — thirst and pale-yellow urine are reliable guides for healthy adults.
Water intake by weight
A common rule of thumb is 30–35 ml per kg of body weight (about half an ounce per pound): 70 kg → 2.1–2.5 liters. It is a starting point, not a prescription; the calculator adjusts it for activity and climate. Larger, more active bodies lose more water through sweat and breathing.
Recommended water intake by age
Needs change little between 19 and 70, but thirst becomes a weaker signal with age, so older adults benefit from scheduled drinking. Children and teens have lower absolute needs (about 1.7–2.6 liters total water); pregnancy raises the adequate intake to about 3.0 liters and breastfeeding to 3.8 liters.
Water and weight loss
Drinking water before meals can slightly reduce intake at that meal, and replacing sugary drinks with water cuts calories directly — that is where the “water helps you lose weight” effect comes from. Water itself does not burn meaningful calories.
Signs you are not drinking enough
Dark yellow urine, headache, fatigue, dry mouth and dizziness on standing. In hot weather or long workouts add 400–800 ml per hour of exercise and include some sodium if you sweat heavily — see the electrolyte drinks guide.
Sources
- National Academies, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate (2005) — https://www.nationalacademies.org/projects/HMD-FNB-19-P-139/publication/10925
- Mayo Clinic, Water: how much should you drink every day? — https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/water/art-20044256
- NIH, DRI summary table (water AI) — https://www.ncbi.nlm.nih.gov/books/NBK56068/table/summarytables.t4/
This calculator is educational and not medical advice. If you have a health condition, are pregnant, or are under 18, ask a clinician before changing your diet.
What the result tells you
Roughly how much total fluid to aim for on an ordinary day. It's a starting line, not a strict quota — food and other drinks count toward it too.
How to use it
- Spread it across the day instead of drinking it all at once.
- Add more when it's hot, when you train, or if you're pregnant or breastfeeding.
- Pale-straw urine is a quick sign you're on track.
Recommendations
- Keep a bottle in sight — visibility is a bigger driver than willpower.
- Water-rich foods like fruit, vegetables, soup, and yoghurt all add to your total.
- Thirst lags behind need in older adults, so don't rely on it alone.
Health & wellbeing
If you have a heart or kidney condition, or a doctor has set a fluid limit, follow their advice over this estimate.
Troubleshooting
Result feels too high to drink? Remember food and other drinks count. Downing very large amounts quickly can be harmful — sip steadily instead.
How it’s calculated
A FitNutrition weight-based estimate. Baseline is ~30–40 mL of fluid per kg of body weight per day (e.g. 70 kg → about 2.1–2.8 L). Increase intake above this only when you have extra losses — training, hot weather, illness, or pregnancy/breastfeeding. Food and other drinks count toward the total. This is a general estimate, not a medical requirement; people with heart, kidney, or fluid-restriction conditions should follow their clinician's advice.
Common questions
Does coffee count toward water intake?
Yes. Caffeinated drinks contribute to total fluid; the diuretic effect is small at normal intakes.
Can you drink too much water?
Yes — very large volumes in a short time can dilute blood sodium (hyponatremia), mainly a risk in endurance events.
How much water on a workout day?
Your baseline plus roughly 0.5 liter per 30 minutes of sweaty exercise.