Water Intake Calculator

A common evidence-based target is 30–35 ml of water per kilogram of body weight per day. For a 70 kg adult that is about 2.1–2.45 liters (roughly 71–83 fl oz). The U.S. Institute of Medicine recommends about 3.7 L/day for men and 2.7 L/day for women as total water (including food). Our calculator combines your weight, exercise, and climate to give you a personalized daily target.

The most widely used formula is weight-based: multiply your weight in kg by 30–35 ml (or drink roughly half your body weight in pounds as ounces). This baseline is then adjusted upward for exercise (about 700 ml per hour), hot/humid weather (about 500 ml per day), and pregnancy or breastfeeding. No single formula is perfect — official IOM/EFSA values are population averages, so individual needs vary.

Add roughly 700 ml (about 24 fl oz) of fluid per hour of moderate exercise to your baseline. For intense or long sessions, the American College of Sports Medicine recommends about 200–300 ml every 10–20 minutes during exercise, and after heavy sweating, replace about 1.2–1.5 liters of fluid per kilogram of body weight lost. Sports drinks add value when sessions exceed 60–90 minutes.

Yes. High temperatures and humidity increase sweat rate, so add roughly 500 ml (about 17 fl oz) per day to your normal target in hot or humid conditions, and more if you are active outdoors. Watch for signs of dehydration — dark urine, thirst, headache, or dizziness — and hydrate before, during, and after time in the heat.

The Institute of Medicine recommends about 3.0 liters (≈100 fl oz) of total daily fluids during pregnancy and about 3.8 liters (≈128 fl oz) while breastfeeding to support milk production. Our calculator applies these targets automatically when you select the corresponding life stage.

The "8×8" rule — eight 8-ounce glasses, or about 1.9 liters — is a memorable guideline but not based on strong evidence. For many adults it is on the low side, since weight-based formulas (30–35 ml/kg) typically suggest 2.1–2.5+ liters. Use it as a starting point, but adjust for your weight, activity, and climate — which is exactly what this calculator does.

Yes. Despite the old myth, caffeinated beverages like coffee and tea do count toward hydration, though water is still the healthiest choice. About 20–30% of daily water typically comes from food — fruits, vegetables, soups, and other moist foods. Official IOM/EFSA values are totals that include all drinks and food moisture; our headline estimate focuses on the fluids you should drink.

Yes. The calculator is 100% free with no registration, and every calculation runs entirely in your browser — no data is ever sent to a server. If you choose to save results, they are stored only in your browser's localStorage, and you can export, import, or clear your history at any time.