Daily Hydration Calculator — educational

Enter your weight, age, and sex, pick an activity level and climate, and see the daily water intake target in litres, fluid ounces, and cups. The base formula is the 33 mL per kg of body weight heuristic the IOM and EFSA "Adequate Intake" range brackets, with an upward adjustment for physical activity and a hot climate, and a small downward adjustment for women and for older adults (the body-water fraction drops with age). The calculator also emits a drink-through-the-day schedule — six suggested cup pours from waking to evening. Nothing leaves your browser.

kg
Daily water target
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In fluid ounces
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US fl oz per day
In cups
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US cups per day (8 fl oz)

How the target is built

The breakdown shows where the litres come from. The base is the weight-only heuristic; the activity and climate columns are the upward adjustments; the age × sex column is the (sometimes downward) percentage nudge for the IOM life-stage brackets and the EFSA per-sex AI.

Source
Litres / day
Notes
Base (33 mL × weight)
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The weight-only baseline.
Activity adjustment
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Climate adjustment
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Age × sex adjustment
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Suggested drink schedule

Six pour points across a normal waking day. The volumes add up to about 90–95 % of the daily target; the remainder comes from food-water (fruits, soups, cooked grains) and the small daily variance a fixed schedule can't capture. Active and very-active users get an extra "during workout" pour.

How this calculator works
  • Base formula. base = 33 mL × weight (kg) — a midpoint of the 30–35 mL/kg range the popular "drink half your bodyweight in ounces" rule approximates. For a 70 kg adult the base is 2.3 L/day, which sits between the EFSA's 2.0 L (women) and 2.5 L (men) per-day Adequate Intake for beverages.
  • Activity adjustment. +0.3 L/day for light exercise up to +1.4 L/day for "very active". The ACSM 2007 guideline recommends 0.4–0.8 L per hour of exercise; the page's per-day value bakes in a 1-hour average for active users.
  • Climate adjustment. +0.5 L/day in hot weather and +1.0 L/day in very hot / arid climates (desert, humid subtropical), reflecting the additional insensible water loss from perspiration.
  • Age × sex adjustment. A small downward nudge for women (−10 %), for adults 56–70 (−5 %), and for seniors over 70 (−15 %). These align with the IOM 2004 Adequate Intake life-stage brackets and the EFSA per-sex AI.
  • Why the schedule is a guide. The six pour points are a habit scaffold, not a prescription. Sip to thirst in addition to the schedule; if your urine is consistently dark yellow, drink more; if it's almost clear, you're well-hydrated.