Institute of Medicine recommendations:

Adult Men:
  Total water: 3.7 L/day (125 oz)
  From beverages: ~3.0 L (13 cups)
  From food: ~0.7 L (20%)

Adult Women:
  Total water: 2.7 L/day (91 oz)
  From beverages: ~2.2 L (9 cups)
  From food: ~0.5 L (20%)

Pregnant Women: +0.3 L/day
Breastfeeding: +0.7 L/day

Children by age:
  1–3 years:   1.3 L/day
  4–8 years:   1.7 L/day
  9–13 (boys): 2.4 L/day
  9–13 (girls): 2.1 L/day
  14–18 (boys): 3.3 L/day
  14–18 (girls): 2.3 L/day

These are total water intakes from all sources (food and beverages), not just drinking water. Approximately 20% of total water intake comes from foods — especially fruits, vegetables, soups, and dairy.

Simple formula: 35 mL per kg body weight

Examples:
  50 kg person: 50 × 35 = 1,750 mL (1.75 L)
  70 kg person: 70 × 35 = 2,450 mL (2.45 L)
  90 kg person: 90 × 35 = 3,150 mL (3.15 L)
  110 kg person: 110 × 35 = 3,850 mL (3.85 L)

Alternatively:
  0.5–1.0 oz per pound of body weight
  70 kg = 154 lbs
  154 × 0.5 oz = 77 oz (2.3 L)
  154 × 0.67 oz = 103 oz (3.0 L)

Adjust for activity level, climate,
body composition, and health status.

The 35 mL/kg formula is a convenient clinical estimate used by dietitians. It typically falls within the IOM adequate intake range for most adults and is easy to calculate without tools.

General exercise guidelines:
  Before: 5–7 mL/kg 4 hrs before
  During: 0.4–0.8 L per hour
  After: 1.5 L per kg of weight lost

Sweat rate estimation:
  Weigh yourself before and after 1 hour
  of exercise (without drinking):
  Sweat rate (L/hr) = pre − post weight (kg)

Example:
  Pre-exercise: 75.0 kg
  Post-exercise: 74.0 kg
  Sweat rate: 1.0 L/hr
  → Drink ~750–1,000 mL during next
    1-hour session

Electrolyte replacement:
  Exercise >60 min in heat:
  400–800 mg sodium/hour recommended

Thirst is a reliable hydration guide for most low-to-moderate exercise. In high heat, long duration (>90 min), or when wearing heavy equipment, proactive drinking is important as thirst lags behind actual fluid needs.

Hot climate adjustment:
  Add 500–1,000 mL per day
  for every 10°C above 25°C

High altitude (>2,500 m):
  Increase by 500–1,500 mL/day
  (faster respiration increases
  respiratory water loss)

Dry environments (indoor heat/AC):
  Add 200–500 mL per day

Air travel:
  Add 200–500 mL per 2 hrs of flight
  (cabin humidity ~15–20%)

Fever:
  Add 100–150 mL per 1°C above
  normal body temperature

Diarrhea/vomiting:
  Replace fluid loss oz-for-oz;
  include electrolytes (ORS formula)

Environmental conditions dramatically affect total water needs. A construction worker outdoors in 35°C heat may need 6–8 L/day, while an office worker in a climate-controlled building needs 2–3 L. Always adjust for conditions, not just body weight.