Hydration Schedule Calculator
Calculate how much water to drink each hour based on your body weight, activity duration and temperature. Includes a bar chart of hourly targets.
How to use this tool
- Enter body weight, hours of activity and temperature in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your target per hour and the full breakdown beneath it.
Adequate hydration supports performance, cognition and thermoregulation. General guidelines recommend 30–40 ml/kg/day at rest; hot conditions and activity increase needs further.
Not medical advice. Individual needs vary. Drink to thirst and consult a doctor if you have kidney or heart conditions.
⚠ This tool is for general information and education only and is not medical advice. Consult a qualified healthcare professional before making health decisions.
Formula
Base per hour (ml) = bodyWeight (kg) × 35 ÷ 24
Heat factor = 1 + max(0, (temperature − 20) ÷ 5) × 0.10
Target per hour = round(Base per hour × Heat factor)
Total = Target per hour × hours of activity
How it works
This calculator derives a per-hour fluid target from a daily baseline of 35 ml per kilogram of body weight, then adjusts upward by 10% for every 5 °C the ambient temperature exceeds 20 °C. The result is a flat hourly target, which is a practical simplification: in reality sweat rate varies with exercise intensity and individual physiology, so treat the output as a starting guide rather than a clinical prescription.
Worked example
- Body weight = 70 kg. Base per hour = 70 × 35 ÷ 24 = 102.08 ml/hr.
- Temperature = 20 °C. Heat factor = 1 + max(0, (20 − 20) ÷ 5) × 0.10 = 1.0 (no adjustment).
- Target per hour = round(102.08 × 1.0) = 102 ml/hr.
- Activity duration = 8 hours. Total = 102 × 8 = 816 ml = 0.82 L.
102 ml/hr; 816 ml total (0.82 L) over 8 hours.
Common mistakes to avoid
- Entering body weight in pounds instead of kilograms — the base formula uses kg, so lbs inputs inflate the per-hour target by roughly 2.2x.
- Applying the heat-adjusted hourly target uniformly when intensity varies within the session; low-effort warm-up periods need far less fluid than peak exertion intervals.
- Treating the total as a water-only target and ignoring electrolytes — high sweat rates over long sessions deplete sodium, and drinking plain water alone risks hyponatremia.
Key terms
- Daily fluid baseline
- A reference intake of 35 ml per kilogram of body weight per day, commonly used in clinical and sports nutrition guidelines as a starting point for hydration needs.
- Heat adjustment factor
- A multiplier applied to baseline fluid needs to account for increased sweating in warm environments; here, +10% for each 5 °C above 20 °C.
- Sweat rate
- The volume of fluid lost through perspiration per unit time; it increases with temperature, humidity, and exercise intensity but is not individually measured by this tool.
- ml/hr (millilitres per hour)
- The unit used to express a fluid intake target per hour, making it easy to plan drinking intervals during activity.
Frequently asked questions
- Does temperature affect hydration needs?
- Yes — for every 5 °C above 20 °C this calculator adds 10% to the base requirement to account for sweat losses.
- Should I drink all at once or spread it out?
- Spread intake evenly throughout the day. Drinking too much at once can dilute blood sodium (hyponatremia) in extreme cases.