How Daily Water Intake Is Calculated

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This calculator estimates how much water you need each day based on your body weight, activity level, climate, and exercise. It uses the EFSA (European Food Safety Authority) guideline of 30–35 ml per kg of body weight as a baseline, with adjustments for physical activity, climate, and exercise duration.

The popular “8 glasses a day” rule has no scientific basis (Valtin, 2002). Actual needs vary significantly depending on body size, activity, and environment.

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1. Base water needs — calculated from body weight at 35 ml per kg (EFSA 2010 guideline range: 30–35 ml/kg)
2. Activity adjustment — a multiplier applied to the base amount (1.0× for sedentary up to 1.5× for extremely active)
3. Exercise extra — additional fluid to replace sweat losses during exercise (based on ACSM guidelines)
4. Climate extra — additional fluid for hot or humid climates
5. Food/drink split — approximately 20% of total water intake comes from food moisture (EFSA 2010), so the remaining 80% is what you need to actively drink

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Activity multipliers

Exercise fluid replacement

The ACSM (2007) recommends 400–800 ml per hour during exercise. This calculator uses 400 ml per 30 minutes (800 ml/hour), the upper end of the ACSM range, as a conservative estimate for moderate-to-vigorous exercise.

Climate adjustments

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• Body weight — the primary determinant of water needs. Heavier people have more tissue to hydrate and higher metabolic water turnover.
• Activity level — general daily activity excluding dedicated exercise. Higher activity increases insensible water losses (breathing, skin evaporation).
• Climate — hot and humid environments increase sweat losses. EFSA notes their reference values assume “moderate environmental temperature.”
• Daily exercise — dedicated exercise sessions increase fluid needs substantially. ACSM recommends replacing sweat losses during and after exercise.
• Tea/coffee — included for the educational message only. Moderate caffeine intake (3–4 cups/day) does not cause net dehydration (Killer et al., 2014). Your tea and coffee count toward daily fluid intake.

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• Total daily water — total water needed from all sources (food + drinks) in litres per day
• Glasses — how many 250 ml glasses of fluid you need to drink (from the drinks-only portion)
• From drinks (80%) — the amount you need to actively consume as fluids
• From food (20%) — the approximate amount you get from food moisture (fruits, vegetables, cooked foods)
• Exercise extra — additional fluid attributed to your exercise duration
• Climate extra — additional fluid attributed to your climate setting

Common myths

”You need 8 glasses of water a day”

This claim has no scientific basis. Valtin (2002) traced it to a 1945 US Food and Nutrition Board report that recommended “1 ml of water for each calorie of food” (~2.5 litres), but added “most of this quantity is contained in prepared foods.” The qualifying sentence was lost over decades of repetition. Actual needs depend on body size, activity, climate, and diet.

”Coffee and tea dehydrate you”

Moderate caffeine consumption does not cause net dehydration. Killer et al. (2014) found no significant differences in hydration markers between drinking 4 cups of coffee versus 4 cups of water in habitual coffee drinkers. The mild diuretic effect is offset by the fluid content of the beverage.

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• No sex differentiation. EFSA provides separate values (2.5L men, 2.0L women), but this calculator uses a weight-based formula instead, which implicitly accounts for sex differences since men typically weigh more. Users of any sex can use it.
• Exercise extra uses ACSM upper bound. The calculator uses 800 ml/hour (the upper end of the ACSM 400–800 ml/hour range). Individual sweat rates vary from 0.3 to 2.5 L/hour depending on intensity, fitness, and environment.
• Climate adjustments are estimates. No authoritative source provides precise ml additions for different climates. The +500 ml (hot) and +750 ml (hot & humid) values are reasonable approximations.
• 20% food fraction is an average. People who eat lots of fruits and vegetables may get 30%+ of water from food. Those eating mostly dry/processed foods may get less.
• Not a medical tool. This is a general guideline. People with kidney disease, heart failure, or on certain medications should consult their doctor about fluid intake.

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All values verified against calculateWaterIntake() function output and manual arithmetic of the formula.