How Much Water Should You Drink a Day?

By Jesse · Last updated July 16, 2026

The answer depends on your body size, how much you exercise, and the climate you live in. There is no universal number that applies to everyone — but there are evidence-based guidelines that give you a reasonable starting point, and simple signals that tell you whether you are hitting it.

Where does the "8 glasses a day" rule come from?

The "eight 8-ounce glasses" rule has no solid scientific basis. Its likely origin is a 1945 US Food and Nutrition Board recommendation that adults need about 2.5 liters of water per day — but the same document noted that most of that amount is already contained in food. The guideline was widely repeated without that crucial context, and the "8 glasses" version spread from there. A 2002 review in the American Journal of Physiology found no evidence supporting it as a universal recommendation.

What the evidence actually says

The most comprehensive review of water intake requirements comes from the European Food Safety Authority (EFSA), which set Adequate Intake values based on large population surveys:

These are totals from all sources — drinks and food combined. About 20 percent of daily water intake for most people comes from food, particularly fruits and vegetables. The US National Academies of Sciences set similar reference values (3.7 L/day for men, 2.7 L/day for women total), which also include food.

Neither body issues a single fixed daily drinking target because needs vary too much by body size, activity, and environment. A weight-based approach — roughly 30 to 35 ml per kilogram of body weight — is a more useful practical tool. The water intake calculator uses this formula and adjusts for exercise.

How exercise changes your needs

Physical activity increases water loss through sweat significantly. A general estimate is 0.5 to 1 liter of extra fluid per hour of moderate-intensity exercise, though this varies with intensity, temperature, and individual sweat rate. A 60-minute run in warm weather can easily produce a liter of sweat loss or more.

Replacing fluid lost during exercise is important for both performance and recovery. Mild dehydration — as little as 2 percent of body weight — measurably reduces endurance performance and concentration. For most workouts under an hour in moderate conditions, drinking to thirst before and after is sufficient. Longer or more intense sessions in heat benefit from a structured intake strategy.

Does coffee or tea count?

Yes. Despite the widespread belief that caffeine dehydrates you, the mild diuretic effect of caffeine does not offset the fluid it is delivered in. Research consistently shows that moderate consumption of caffeinated drinks contributes to daily hydration. Water, milk, juice, coffee, and tea all count toward your total intake.

How to know if you are drinking enough

The simplest indicator is urine color. Pale yellow (straw-colored) generally means adequate hydration. Dark yellow or amber means drink more. Nearly clear urine can indicate overhydration, which is uncommon but possible. Other signs of mild dehydration include headache, difficulty concentrating, and increased perceived effort during exercise.

Thirst is a real signal but lags slightly behind actual fluid deficit, especially in older adults and during vigorous exercise. Relying on thirst alone is usually fine for sedentary people in comfortable temperatures, but athletes and people working in heat benefit from a more proactive approach.

Factors that raise your needs

• Hot or humid environments — sweat losses increase substantially.
• High altitude — triggers increased respiratory water loss and a mild diuretic response.
• Illness with fever — roughly 150 ml extra per degree Celsius above normal body temperature.
• High fiber intake — fiber needs water to move through the digestive tract without causing constipation.
• Breastfeeding — EFSA estimates an additional 700 ml per day above baseline.

Can you drink too much water?

Yes, though it is rare for healthy adults following normal eating patterns. Drinking very large quantities of plain water in a short period can dilute sodium in the blood to dangerous levels — a condition called hyponatremia. It is most commonly seen in endurance athletes who drink aggressively during long events without replacing electrolytes. For the general population eating a normal diet, the risk of overhydration is essentially zero at any reasonable intake.

These are general guidelines for healthy adults, not medical advice. If you have kidney disease, heart failure, or another condition that affects fluid balance, your doctor or dietitian should guide your fluid intake. See our methodology for the sources behind these recommendations.