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What water does in circulation, digestion and temperature control, where the evidence is strong, and how the weight-based daily target for adults is worked out.
How does water help your body? It transports nutrients into cells, carries metabolic waste out, regulates temperature, and keeps joints moving. Each of those core functions in turn:
Consistently meeting your daily water intake targets has wide-ranging effects on key organ systems, from digestion to cognition.
Why drink more water for physical and mental performance? Because measurable losses start small: the studies below show what 1–2% dehydration does to heart rate, plasma volume, and performance.
Water isn't a weight-loss tool on its own, but the benefits of drinking a lot of water are supporting ones: appetite control, a small metabolic nudge, and workouts you can actually sustain.
From your first glass of the day to your last, fluid balance shapes both immediate wellbeing and longer-term health.
The quickest reliable starting point is body weight: about 35 mL per kilogram per day for adults aged 19–64, adjusted for age, sex, activity, and pregnancy or breastfeeding 8. The tables below summarize those adjustments — the same model our homepage calculator applies to your inputs.
| Age group | Baseline (mL per kg body weight per day) |
|---|---|
| 0–6 months | 150 (from breast milk or formula only — no plain water before 6 months) |
| 7–12 months | 100 |
| 1–3 years | 90 |
| 4–8 years | 70 |
| 9–13 years | 50 |
| 14–18 years | 40 |
| 19–64 years | 35 |
| 65+ years | 30 |
Activity modifiers are shown for adults aged 19–64 (the calculator's activity scale, modelled on ACSM guidance 9); children, teens, and older adults are slightly lower.
| Factor | Adjustment |
|---|---|
| Female sex | × 0.95 (≈ 5% lower baseline, reflecting body composition) 8 |
| Light activity | + 3% |
| Moderate activity | + 7% |
| Intense activity | + 12% |
| Extreme activity | + 20% |
| Pregnancy | + ≈ 300 mL/day, phased smallest in the 1st and largest in the 3rd trimester 7 |
| Breastfeeding (exclusive, early months) | + ≈ 700 mL/day, tapering as nursing becomes partial 7 |
| Person | Baseline | After adjustments |
|---|---|---|
| 70 kg man, moderate activity | 2.45 L/day | ≈ 2.6 L/day |
| 55 kg woman, light activity | 1.93 L/day | ≈ 1.9 L/day |
| 25 kg child (4–8 years) | 1.75 L/day | 1.75 L/day (resting) |
For context, total-water guideline anchors (food + all drinks combined) are 2.0 L/day for women and 2.5 L/day for men (EFSA) 7, and 2.7 L/day for women and 3.7 L/day for men (U.S. IOM) 8 — roughly 80% from drinks and 20% from food.
The water intake by weight table gives the drinks share of those anchors size by size — litres, cups, and glasses from 40–150 kg at rest and at moderate activity, with food moisture on top of those figures.
One ceiling matters as much as the targets: healthy kidneys excrete at most about 1 liter per hour 6, so drinking far more than that in a short window risks hyponatremia (see our guide on how much water is too much).
Explore more hydration topics
Creatine does not add a fixed extra water amount at recommended doses. Hit your normal daily target, spread through the day, and adjust for exercise and heat.
When to drink before, during and after exercise, electrolytes for prolonged activity, sports drinks versus water, and intake matched to sweat rate.
Moderate coffee does not dehydrate regular drinkers, and its fluid counts toward daily intake. Tea and soda compared, and how caffeine tolerance develops.
How much water is too much? Over 3–4 litres in a few hours can outpace healthy kidneys (about 0.8–1 L/hour) and dilute blood sodium. Warning signs, in order.
Heat raises sweat losses and cold weather mutes thirst. How intake changes with temperature, humidity, altitude and seasonal transitions.
No plain water before six months. From six months, small amounts alongside meals, and how to check tap, bottled or filtered water for safety.