The human body is roughly 60% water. Water plays a vital role in regulating body temperature, transporting nutrients, flushing out waste, lubricating joints and supporting cell function. The amount each person needs varies with weight, age, sex, activity level and environmental conditions.
The World Health Organization (WHO) and nutrition bodies recommend around 3.7 liters per day for adult men and about 2.7 liters for adult women. These figures include the water contained in food (roughly 20%). The simplest formula is: body weight × 35 ml = base daily requirement.
Athletes should adjust upward by adding 500 to 1,000 ml for every hour of intense training. In hot and humid conditions, add roughly 500 ml more.
The table below shows the daily water requirement by body weight for someone with a moderate activity level:
| Body Weight (kg) | Base Water (liters) | With Moderate Activity (liters) | Glasses |
|---|---|---|---|
| 50 kg | 1.75 | 2.2 | 8-9 |
| 60 kg | 2.1 | 2.6 | 10-11 |
| 70 kg | 2.45 | 3.0 | 12 |
| 80 kg | 2.8 | 3.5 | 14 |
| 90 kg | 3.15 | 3.9 | 15-16 |
| 100 kg | 3.5 | 4.4 | 17-18 |
| 110 kg | 3.85 | 4.8 | 19 |
Water requirements change across the lifespan. Growing children and teenagers, adults over 65, and pregnant or breastfeeding women all have different needs:
| Age Group / Sex | Daily Requirement (liters) | Note |
|---|---|---|
| Children 4-8 years | 1.2 | Includes water from food |
| Teens 9-13 years | 1.6 - 1.8 | Boys higher than girls |
| Adult men | 3.0 - 3.7 | Depending on activity |
| Adult women | 2.2 - 2.7 | Depending on activity |
| Pregnant women | 3.0 | Additional 300 ml |
| Breastfeeding women | 3.5 - 3.8 | Additional 500 ml |
| Adults 65+ | 2.0 - 2.5 | Thirst sensation is weaker |
The most accurate way to estimate daily water needs is a weight-based formula. The most commonly used ones are:
Dehydration is a real risk, but drinking too much water can also lead to hyponatremia (water intoxication), where blood sodium falls to dangerous levels. For most healthy adults, the safe upper limit for plain water is around 3 to 4 liters per day. The kidneys can process a maximum of roughly 0.8 to 1 liter per hour. Professional athletes may need more under specific conditions, but only under the supervision of a nutrition specialist.
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