8 glasses a day is a myth. Your water needs depend on your weight, activity level, and climate. Get a personalized daily hydration goal based on your body.
A 70 kg (154 lb) adult needs approximately 2.1 to 2.45 liters of water per day, based on the European Food Safety Authority (EFSA) guideline of 30 to 35 ml per kilogram of body weight. This baseline covers water from all sources including food, which typically contributes 20 to 30 percent of total intake.
| Body Weight | Minimum (30 ml/kg) | Recommended (35 ml/kg) |
|---|---|---|
| 50 kg (110 lb) | 1.50 L | 1.75 L |
| 60 kg (132 lb) | 1.80 L | 2.10 L |
| 70 kg (154 lb) | 2.10 L | 2.45 L |
| 80 kg (176 lb) | 2.40 L | 2.80 L |
| 90 kg (198 lb) | 2.70 L | 3.15 L |
| 100 kg (220 lb) | 3.00 L | 3.50 L |
| Factor | Adjustment | Additional Water |
|---|---|---|
| Light exercise (1-3 days/week) | × 1.1 | +200-300 ml |
| Moderate exercise (3-5 days/week) | × 1.3 | +400-600 ml |
| Intense exercise (6-7 days/week) | × 1.5 | +600-1000 ml |
| Hot/humid climate | × 1.2 | +300-500 ml |
| High altitude (>2500m) | × 1.15 | +200-400 ml |
| Pregnancy | +300 ml | +300 ml |
| Breastfeeding | +700 ml | +700 ml |
Sources: EFSA (2010) Scientific Opinion on Dietary Reference Values for Water; U.S. National Academies of Sciences, Engineering, and Medicine (2005).
The most accurate way to estimate your daily water needs is based on body weight. The European Food Safety Authority (EFSA) recommends 30 to 35 ml of water per kilogram of body weight. This formula accounts for differences in body size and provides a personalized baseline that the standard "8 glasses a day" rule cannot match. The table below covers common body weights so you can find your target quickly.
| Weight (kg) | Weight (lbs) | Minimum (30ml/kg) | Recommended (35ml/kg) | Cups (8oz) |
|---|---|---|---|---|
| 45 kg | 99 lb | 1.35 L | 1.58 L | ~6-7 cups |
| 55 kg | 121 lb | 1.65 L | 1.93 L | ~7-8 cups |
| 65 kg | 143 lb | 1.95 L | 2.28 L | ~8-10 cups |
| 75 kg | 165 lb | 2.25 L | 2.63 L | ~9-11 cups |
| 85 kg | 187 lb | 2.55 L | 2.98 L | ~11-13 cups |
| 95 kg | 209 lb | 2.85 L | 3.33 L | ~12-14 cups |
| 105 kg | 231 lb | 3.15 L | 3.68 L | ~13-16 cups |
Your weight in kg × 35 = daily water in ml Your weight in lbs × 0.5 = daily water in oz
Sources: EFSA (2010) Scientific Opinion on Dietary Reference Values for Water; National Academies of Sciences, Engineering, and Medicine (2005).
Any water target based on body weight is a starting estimate, because sweat rate varies roughly threefold between people doing the same workout in the same room. An 80 kg runner may lose 0.8 L per hour while a training partner of the same size loses 2.1 L. The formula gets you close; one cheap measurement gets you exact.
Weigh yourself naked before a hard one-hour session, drink nothing and use no bathroom during it, then weigh again the same way. Every kilogram lost is roughly one litre of sweat, so a 1.2 kg drop means a sweat rate of about 1.2 L/hour. Plan to replace 75-100% of that during longer efforts, sipping in 150-250 mL doses every 15-20 minutes rather than gulping a litre at once — the stomach empties faster with smaller, more frequent volumes.
| Body weight change in 1 hour | Estimated sweat rate | Suggested intake per hour |
|---|---|---|
| 0.5 kg | 0.5 L/h | 0.4-0.5 L |
| 0.8 kg | 0.8 L/h | 0.6-0.8 L |
| 1.2 kg | 1.2 L/h | 0.9-1.2 L |
| 1.8 kg or more | 1.8+ L/h | 1.2-1.5 L plus electrolytes |
Sweat rate climbs sharply above roughly 30°C, and high humidity matters more than temperature alone: sweat that drips off instead of evaporating cools nothing, so you dehydrate while still feeling soaked. Altitude adds respiratory water loss and increases urine output. On hot days, aim for the top of your replacement range and add sodium; on cool days in the same session type, the low end is plenty.
Plain water is not always the right answer. Sweat carries roughly 300-1,500 mg of sodium per litre, and if you finish workouts with white salt streaks on dark clothing you are at the salty end — a pinch of salt in your bottle or an electrolyte tab is reasonable. The genuine danger in long events is the opposite of dehydration: drinking far beyond thirst dilutes blood sodium (hyponatremia), which can be life-threatening. Never exceed about one litre per hour for hours on end without sodium, and use urine colour as a daily check — pale straw is the target, clear as water means you have overdone it.
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