How much water should you drink daily? A science-based guide
The eight-glasses-per-day rule is everywhere, but it is not backed by strong science. Your actual water needs depend on your body size, activity level, climate, and diet. Here is how to figure out how much water you really need.
The science behind water requirements
The National Academies of Sciences recommends about 3.7 liters (125 ounces) per day for men and 2.7 liters (91 ounces) per day for women. But these are total water intake recommendations, including water from food. Food provides roughly 20 percent of your daily water, meaning you need about 3 liters (100 ounces) from beverages for men and 2.2 liters (74 ounces) for women.
These numbers are starting points, not rigid targets. Your actual needs vary day to day based on several factors.
Factors that increase your needs
Exercise increases water loss through sweat and respiration. For every 30 minutes of moderate exercise, add 12 to 16 ounces of water. For intense exercise in hot conditions, add 20 to 30 ounces per hour.
Hot or humid weather increases sweat rate significantly. Even if you do not feel sweaty, you lose water through evaporation. In hot climates, add 16 to 32 ounces per day.
Altitude above 8,000 feet increases urination and respiration rates, leading to faster dehydration. Add 16 to 32 ounces per day when at high altitude.
Pregnancy and breastfeeding increase water needs significantly. Pregnant women need an additional 8 to 12 ounces per day. Breastfeeding women need 24 to 32 extra ounces to support milk production.
Illness with fever, vomiting, or diarrhea dramatically increases water loss. In these cases, drink to thirst and consider electrolyte replacement.
How to calculate your personal target
A reasonable formula is your body weight in pounds divided by two, giving you the target ounces per day as a baseline. A 180-pound person would target 90 ounces per day. Then add 12 ounces for every 30 minutes of exercise and adjust for climate.
This formula is simple and accounts for the most significant variable — body size. Use the Water Intake Calculator to get a personalized recommendation based on your full profile.
Signs you are dehydrated
Dark urine is the most accessible indicator. Pale yellow means adequate hydration. Dark yellow or amber means you need water. Thirst is a late signal — by the time you feel thirsty, you are already mildly dehydrated.
Other signs include headache, fatigue, dry mouth, reduced urine output, and difficulty concentrating. Chronic low-level dehydration is common and often goes unnoticed.
Can you drink too much?
Overhydration is rare but dangerous. Drinking massive amounts of water without electrolytes can cause hyponatremia, where blood sodium drops to dangerously low levels. This is most common in endurance athletes who drink excessive water without replacing sodium.
For most people, drinking to thirst and checking urine color is sufficient. Your body has excellent regulatory systems for water balance. The Water Intake Calculator helps you find your range, but listening to your body is the ultimate guide.