About

What this site is

How Much Water Do I Need is a free, science-based calculator for personalized daily water-intake recommendations. You enter your weight, age, sex, activity level, pregnancy or breastfeeding stage, and climate; the calculator combines them with published health guidelines to estimate how much water you should drink per day, and it recalculates live as you adjust the inputs. The site also offers a private water tracker and plain-language hydration guides.

All calculations run entirely in your browser: there are no accounts, and the health information you enter never leaves your device. The one exception is the optional climate-detection feature: if you enable it, your browser asks for your location and sends rounded coordinates (about 1 km of precision) to the Open-Meteo weather service to look up local weather and elevation for the climate adjustment. None of your health inputs are included in that request. See our Privacy Policy for details.

How the recommendation is calculated

The estimate starts from body weight. Each age group has a baseline of milliliters of water per kilogram of body weight per day:

• 0–6 months — 150 mL per kg of body weight per day
• 7–12 months — 100 mL per kg of body weight per day
• 1–3 years — 90 mL per kg of body weight per day
• 4–8 years — 70 mL per kg of body weight per day
• 9–13 years — 50 mL per kg of body weight per day
• 14–18 years — 40 mL per kg of body weight per day
• 19–64 years — 35 mL per kg of body weight per day
• 65+ years — 30 mL per kg of body weight per day

Female sex multiplies the baseline by 0.95 (≈5% lower, reflecting body composition). Physical activity adds 3% for light, 7% for moderate, 12% for intense, and 20% for extreme activity for adults aged 19–64; every other age group has its own age-specific activity scale.

Pregnancy and breastfeeding are applied as percentages of the adjusted baseline, so they scale with body weight. Pregnancy adds 10%, phased by trimester (70% of that in the 1st, 100% in the 2nd, and 130% in the 3rd — roughly 160–300 mL/day for a 70 kg woman). Breastfeeding adds 20%, scaled by nursing stage (125% of that while exclusively nursing in the early months, 100% for established exclusive nursing, 75% and 50% for high and low partial nursing, and 25% at weaning — roughly 465–580 mL/day for a 70 kg woman while exclusively nursing). The published guidelines these adjustments are modelled on amount to ≈300 mL/day during pregnancy and ≈700 mL/day while exclusively breastfeeding (IOM/EFSA/WHO). Hot weather adds a further climate adjustment, unless the safety check has flagged the result as dangerous, in which case the climate adjustment is skipped.

The Methodology page walks through the same calculation step by step, with a worked example, the input validation ranges, and how the underlying guidance is maintained.

Safety guardrails

Every result is capped by safety limits: the drinking-water (fluid) recommendation never exceeds 220% of the weight-based baseline — an increase of at most 120% — which guards against hyponatremia (water intoxication). The displayed total-water figure, which includes the roughly 20% share of water that normally comes from food, is derived from that capped fluid amount. The calculator recommends medical consultation during pregnancy or breastfeeding. A separate safety check also compares each result with the minimum safe intake for the age group and with extreme-weight and medical-condition risk rules; in the current implementation it shows the same warning whenever any extra factor is selected, and when a result falls 10% or more below the safe minimum or another risk finding requires it. The check runs on the figure before the optional climate adjustment and is not rerun afterwards. The adjustments from selected factors are capped per category (health +20%, lifestyle +15%, dietary +15%). As a physiological anchor: healthy kidneys excrete at most about 1 liter of fluid per hour, so intake above that rate is what creates risk.

Key hydration facts

• EFSA adequate intake for total daily water (all sources, including food): 2.0 L/day for women and 2.5 L/day for men.
• U.S. Institute of Medicine adequate intake for total daily water: 2.7 L/day for women and 3.7 L/day for men.
• Roughly 80% of total water intake normally comes from drinks and 20% from food (IOM).
• A standard glass is 355 mL (12 fl oz).

Where the guidance comes from

• Institute of Medicine (now the National Academies): Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2004).
• EFSA Panel on Dietetic Products, Nutrition, and Allergies: Scientific Opinion on Dietary Reference Values for Water (2010).
• WHO: guidance on infant and young child feeding and safe drinking water.
• American College of Sports Medicine Position Stand: Exercise and Fluid Replacement (2007).

Who we are

The site is built and maintained by webuildstuff.io. Feedback and corrections are welcome through the contact page — see also our Privacy Policy and Terms of Service.

The calculator provides general estimates for healthy people and is not medical advice. If you have a medical condition, are pregnant, or care for an infant, talk to a healthcare professional about your specific needs.