Most adults need roughly 2.7 to 3.7 liters (91 to 125 ounces) of total water per day, but about 20% of that comes from food. That means you’re looking at around 9 to 13 cups of beverages daily, depending on your size, activity level, and environment. The old advice to drink eight glasses a day isn’t wrong as a starting point, but it isn’t based on any real science either.
The “8 Glasses a Day” Rule Isn’t Evidence-Based
A widely cited review in the American Journal of Physiology searched for scientific support behind the advice to drink at least eight 8-ounce glasses of water daily and found none. Surveys of thousands of healthy adults showed most people were already well-hydrated without hitting that specific target. The review also confirmed that caffeinated drinks and even mild alcoholic beverages like beer (in moderation) count toward your daily total, contrary to popular belief.
That said, the review was careful to note this applies to healthy adults in temperate climates living relatively sedentary lives. If you exercise regularly, work outdoors, or live somewhere hot or at high elevation, you genuinely do need more.
A Simple Way to Estimate Your Needs
One straightforward formula used in clinical settings: multiply your body weight in kilograms by 30 milliliters. A 70 kg (154-pound) person would need about 2,100 mL, or roughly 9 cups of fluid per day. A 90 kg (198-pound) person would need closer to 2,700 mL, or about 11.5 cups.
If you don’t think in kilograms, divide your weight in pounds by 2.2 first. This formula gives you a baseline for a typical day without heavy exercise or heat exposure. It includes all fluids: water, coffee, tea, juice, milk, and the water content in your meals. Since food covers about 20% of your total intake, you can subtract that portion and focus on drinking the remaining 80%.
How Exercise Changes Your Needs
During physical activity, your body loses water through sweat at rates that vary dramatically from person to person. The American College of Sports Medicine recommends keeping fluid loss below 2% of your body weight during exercise, because beyond that threshold, your physical and mental performance starts to decline noticeably.
Rather than following a one-size-fits-all drinking schedule, the most reliable approach is to weigh yourself before and after a workout. Every pound lost represents roughly 16 ounces of fluid you need to replace. Over time, this gives you a personalized sense of how much you sweat during different activities and conditions, which is far more useful than a generic guideline. As a rough starting point, most people lose between 0.5 and 1.5 liters per hour of moderate exercise.
Heat, Humidity, and Altitude
Hot weather increases sweat rates, but altitude is an underappreciated factor. At moderate elevations (up to about 13,000 feet), men can lose up to 1,900 mL of extra water per day just through breathing, and women up to 850 mL. Urinary losses also increase by roughly 500 mL daily at altitude because your body adjusts its blood chemistry to cope with lower oxygen. Combined, that’s potentially an extra liter or more per day that you wouldn’t need at sea level.
For active mountain sports, recommendations run as high as 400 to 800 mL per hour, with added sodium to help your body retain the fluid. If you’re hiking, skiing, or climbing at elevation, your usual drinking habits almost certainly won’t keep up with the demand, especially since cold, dry air can suppress your sense of thirst.
How to Tell If You’re Drinking Enough
Your body has a remarkably precise system for regulating water balance. Thirst is the most obvious signal, and for healthy adults it works well. The simplest hydration check is urine color: pale yellow means you’re well-hydrated, dark yellow or amber means you need more fluid. Clear and colorless can actually mean you’re overdoing it.
Mild dehydration, defined as losing just 1% to 3% of your body weight in water, shows up as thirst, dry mouth, and fatigue before anything more dramatic happens. Your heart rate may increase slightly, and you might notice it’s harder to concentrate. For a 160-pound person, 1% loss is less than 2 pounds of water, which is easy to reach on a busy day when you simply forget to drink.
You Can Drink Too Much
Your kidneys can process about one liter of fluid per hour. Drinking significantly more than that over several hours can dilute the sodium in your blood to dangerous levels, a condition called hyponatremia. It’s rare in everyday life but does occur in endurance athletes and occasionally in people who force themselves to drink far beyond thirst in the belief that more water is always better.
The practical takeaway: sipping steadily throughout the day is more effective than gulping large amounts at once. Your body absorbs and uses water better when intake is spread out, and you avoid putting unnecessary strain on your kidneys.
What Counts Toward Your Total
Plain water is the simplest choice, but it’s not the only one. Coffee, tea, sparkling water, milk, juice, soup, and even watery foods like watermelon, cucumbers, oranges, and yogurt all contribute to your daily fluid intake. The long-standing belief that coffee dehydrates you has been largely debunked. Caffeine has a mild diuretic effect, but the water in a cup of coffee more than compensates for it.
Sugary drinks and alcohol in large quantities are the main exceptions. Alcohol above moderate amounts acts as a stronger diuretic and can leave you with a net fluid loss. Sugary beverages technically hydrate you but come with enough downsides that they’re not worth relying on as a primary fluid source.