How Much Water Should I Drink Per Day in Hot Weather?
Calculate exactly how much water you need per day based on your weight, temperature, and exercise minutes. Uses the medical 35 mL/kg formula with heat and activity adjustments. Instant results in liters and glasses.
See step-by-step calculation
When to use this calculator
- A runner training outdoors in summer at 34°C wants to know exactly how many liters to carry and drink throughout the day.
- A construction worker spending 8 hours on-site in a tropical climate needs a reliable daily fluid target to avoid heat exhaustion.
- A parent planning a beach day with kids wants to estimate how much water the whole family should bring.
- A hiker preparing for a multi-day trek in a desert environment needs to calculate per-day water requirements for resupply planning.
Daily Water Needs by Temperature & Exercise (70 kg person, 35 mL/kg baseline)
| Temperature | No exercise | 30 min exercise | 60 min exercise |
|---|---|---|---|
| 20°C | 2.45 L (10 glasses) | 3.0 L (12 glasses) | 3.5 L (14 glasses) |
| 25°C | 2.45 L (10 glasses) | 3.0 L (12 glasses) | 3.5 L (14 glasses) |
| 28°C | 2.82 L (11 glasses) | 3.3 L (13 glasses) | 3.8 L (15 glasses) |
| 32°C | 3.07 L (12 glasses) | 3.6 L (14 glasses) | 4.1 L (16 glasses) |
| 35°C | 3.19 L (13 glasses) | 3.7 L (15 glasses) | 4.2 L (17 glasses) |
| 38°C | 3.43 L (14 glasses) | 3.9 L (16 glasses) | 4.5 L (18 glasses) |
| 40°C | 3.55 L (14 glasses) | 4.1 L (16 glasses) | 4.6 L (18 glasses) |
Fuente: ACSM Exercise and Fluid Replacement Position Stand (2007) + EFSA Dietary Reference Values for Water (2010). Formula: Base = 70 × 35 mL; heat multiplier = 1 + (T−25)/5 × 0.15 for T > 25°C; exercise addition = minutes × 17 mL. Glasses calculated at 250 mL each.
How it works
How much water to drink per day in the heat: the formula
This calculator uses a three-component formula based on recommendations from sports medicine and nutrition literature:
1. Baseline water requirement
Base (mL) = body weight (kg) × 35The 35 mL/kg baseline is widely cited in clinical nutrition and represents the average daily water requirement for a sedentary adult in a temperate environment. The EFSA (European Food Safety Authority) and ACSM both anchor adult recommendations near this figure.
2. Heat adjustment (temperatures above 25°C)
Adjusted base = Base × (1 + (T − 25) / 5 × 0.15)For every 5°C above 25°C, fluid needs increase by approximately 15%. At 35°C (10°C above threshold), the multiplier is 1.30 — a 30% increase over baseline.
| Temperature | Multiplier | Water (70 kg, no exercise) |
|---|---|---|
| 25°C | 1.00 | 2.45 L |
| 30°C | 1.15 | 2.82 L |
| 35°C | 1.30 | 3.19 L |
| 40°C | 1.45 | 3.55 L |
3. Exercise addition
Exercise (mL) = exercise minutes × 17The ACSM recommends approximately 500 mL per 30 minutes of moderate exercise — equivalent to ~17 mL/min.
| Exercise duration | Water to add |
|---|---|
| 30 minutes | +510 mL |
| 45 minutes | +765 mL |
| 60 minutes | +1,020 mL |
| 90 minutes | +1,530 mL |
Full formula
Total (mL) = [weight × 35 × (1 + max(0, T−25)/5 × 0.15)] + [minutes × 17]
Liters = Total / 1000
Glasses = round(Total / 250)Quick reference table (70 kg person)
| Temperature | No exercise | 30 min exercise | 60 min exercise |
|---|---|---|---|
| 20°C | 2.45 L (10 glasses) | 3.0 L (12 glasses) | 3.5 L (14 glasses) |
| 28°C | 2.82 L (11 glasses) | 3.3 L (13 glasses) | 3.8 L (15 glasses) |
| 32°C | 3.07 L (12 glasses) | 3.6 L (14 glasses) | 4.1 L (16 glasses) |
| 38°C | 3.43 L (14 glasses) | 3.9 L (16 glasses) | 4.5 L (18 glasses) |
Important notes
This formula gives a daily target for fluid intake from all sources, including water, drinks, and the water content of food (which typically accounts for 20–30% of total intake). If you eat little solid food (e.g. fasting), increase your drinking water target accordingly.
This calculator is a planning tool. For medical conditions affecting fluid balance (kidney disease, heart failure, hyponatremia risk), consult a healthcare professional.
Hydration schedule tip
Do not rely on thirst alone in hot conditions — by the time you feel thirsty you may already be 1–2% dehydrated, which reduces physical and cognitive performance. Space your intake: 500 mL in the morning on waking, then roughly equal portions each hour.
Worked example: runner on a hot day
Frequently asked questions
How much water should I drink per day in 30°C (86°F) heat?
Why does this calculator give more than the standard '8 glasses a day'?
How does high temperature increase my water needs?
How much extra water do I need per hour of exercise?
Should I drink the entire amount as plain water?
What urine color indicates good hydration?
Do sports drinks replace plain water in hot weather?
Can drinking too much water be dangerous?
How do children's hydration needs compare to adults?
Does humidity affect hydration needs beyond temperature?
Sources & references
- American College of Sports Medicine (ACSM) — Exercise and Fluid Replacement Position Stand
- European Food Safety Authority (EFSA) — Dietary Reference Values for Water
- National Academies of Sciences — Dietary Reference Intakes for Water and Electrolytes
- Centers for Disease Control and Prevention (CDC) — Water and Healthier Drinks
Methodology & trust
Calculadora de salud revisada por el equipo editorial de Hacé Cuentas, contrastada con American College of Sports Medicine (ACSM) — Exercise and Fluid Replacement Position Stand, según nuestra política editorial y metodología.
Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). How Much Water Should I Drink Per Day in Hot Weather?. Hacé Cuentas. https://hacecuentas.com/hydration-hot-climate-daily-activity
Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.