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Daily Water Intake Calculator

Methodology

How we calculate your number

Your result is total water need minus water from food, built from seven additive terms. Each one is shown in the calculator's "Why this number?" panel so nothing is a black box.

1

Baseline need

weight (kg) × ml-per-kg for your age band

We use roughly 29–40 ml/kg/day depending on age (higher for children and teens, lower for adults over 65), which sits inside the range used clinically. This is cross-checked against EFSA's Adequate Intake for total water (2.0 L/day women, 2.5 L/day men) and the U.S. National Academies figure (2.7 L/day women, 3.7 L/day men).

2

Body composition adjustment

+8% of baseline for male sex at birth

On average, higher lean muscle mass holds more water relative to body weight, which is reflected in the differing EFSA/IOM reference values between sexes.

3

Exercise replacement

exercise minutes ÷ 60 × sweat rate (0.4–1.0 L/hr)

Based on ACSM position-stand sweat-rate ranges: light activity ≈0.4 L/hr, moderate ≈0.65 L/hr, vigorous ≈1.0 L/hr. This is calculated from actual minutes you enter, not a flat "active/inactive" bucket.

4

Climate load

climate band (0–0.65 L) × humidity factor (0.85×–1.25×)

Heat raises sweat loss; humidity changes how efficiently that sweat evaporates and cools you, so dry heat and humid heat are weighted differently rather than a single "hot" setting.

5

Altitude

+0.3 L per 1,000 m above 2,500 m (capped at 1.0 L)

Above roughly 2,500 m, faster breathing in thin, dry air and altitude-related diuresis both increase fluid loss.

6

Pregnancy & breastfeeding

+0.3 L (pregnant) or +0.7 L (breastfeeding)

Matches EFSA's published additional intake recommendations for these life stages.

7

Water from food

−20% of your baseline term

Food supplies roughly a fifth of total water intake on a typical diet (more with fruit- and vegetable-heavy diets). We subtract this only from the baseline term — sweat losses from exercise and heat still need to be replaced by drinking.

Why a range, not one number?

We display your target as a range (roughly ±8%) rather than a single decimal. Hydration science does not support a false-precision figure like "2.647 L" — individual variation in metabolism, diet composition and non-exercise activity is real, so the calculator gives you a sensible band to aim for instead.

Sources

  • EFSA Panel on Dietetic Products, Nutrition and Allergies (2010). "Scientific Opinion on Dietary Reference Values for water." EFSA Journal.
  • Institute of Medicine (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
  • Sawka, M.N. et al. (2007). "Exercise and Fluid Replacement." American College of Sports Medicine position stand. Medicine & Science in Sports & Exercise.
  • Maughan, R.J. et al. (2016). "A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index."American Journal of Clinical Nutrition.

This calculator provides general educational estimates and is not medical advice. Individual needs vary. Speak to a doctor before making changes if you are pregnant, or have kidney, heart or liver conditions, or take medication affecting fluid balance.

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