How the calculation works
A common starting point for adults is about 35 ml of fluid per kilogram of body weight per day (about half an ounce per pound); for a 70 kg (154 lb) person that is roughly 2.5 liters. On top of that, the calculator adds 12 ml per minute of exercise (about 0.7 liters per hour) for fluid lost through sweat, and about 0.5 liters a day for hot, humid weather.
The result is your total fluid intake across the day. The glass count is rounded up based on a standard 250 ml (about 8 oz) glass.
Where your water comes from
Only part of your fluid needs to come from plain water. Tea, coffee, milk, soup and water-rich fruit and vegetables all count; in a normal diet about 20% of fluid comes from food. Caffeinated drinks in moderate amounts do not dehydrate you, but do not count sugary drinks or alcohol as water.
Spread your water across the day instead of drinking it all at once. A simple routine such as a glass with every meal plus one before and after exercise works for most people.
How to tell if you drink enough
The most practical indicator is urine color: pale straw yellow means you are well hydrated, dark yellow means you should drink more. Thirst, headache, fatigue, trouble concentrating and constipation can also be signs of mild dehydration.
Drinking enough water also helps when losing weight: water before a meal can increase fullness and keeps you from mistaking thirst for hunger. But water does not burn fat on its own and does not replace a calorie deficit.
Limitations
This is a general estimate for healthy adults. Long endurance sessions, high altitude, fever, vomiting or diarrhea raise fluid needs significantly. During pregnancy and breastfeeding, needs are roughly 0.3-0.7 liters a day higher.
Drinking too much can also be harmful: drinking several liters in a short time can lower blood sodium to dangerous levels. During long workouts you may need electrolytes, not just water.
When to see a doctor
If you have kidney, heart or liver disease or take medication such as diuretics, your doctor may have recommended a fluid limit; follow that advice rather than this calculator. Constant excessive thirst with frequent urination can be a sign of diabetes. Seek medical help for signs of serious dehydration such as dizziness, confusion or passing very little urine.