Hydration Calculator
Estimate daily hydration in litres, ounces, and cups from weight, activity, and climate.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this hydration calculator
- Pick your preferred weight unit (kg or lb) and enter your body weight.
- Choose the activity level that best matches your typical day.
- Select your local climate (temperate, warm, or hot).
- Click Calculate to see your daily hydration target in litres, fluid ounces, and cups.
- Use Copy to save the result or Reset to start over with new inputs.
About this hydration calculator
The Hydration Calculator estimates how much fluid an adult should drink each day, starting from a research-backed baseline of about 33 ml per kilogram of body weight, then adding extra fluid for exercise and heat. Activity adds 5 to 20 ml per kg depending on training duration, and a warm or hot climate adds another 350 to 700 ml to replace sweat losses. For example, a 70 kg moderately active person in temperate weather has a baseline of 2,310 ml plus a 700 ml activity adjustment, giving roughly 3.0 L per day — about twelve 250 ml cups. The formula mirrors fluid-balance recommendations adapted from the Institute of Medicine adequate intake tables and ACSM hydration guidance. Total water from food and other beverages counts too, so plain water alone need not hit this number. This tool is for general education and is not medical advice — consult a qualified healthcare professional before changing fluid intake if you have kidney, heart, liver, or hormonal conditions.
FAQ
- Where does the 33 ml/kg baseline come from?
- It is a clinical rule of thumb for healthy adults derived from the Institute of Medicine adequate intake tables (≈ 2.7 L/day for women, 3.7 L/day for men of average weight) and reflects total water from beverages and food.
- Does coffee or tea count toward my target?
- Yes. Despite their mild diuretic effect, caffeinated drinks consumed in normal amounts still produce a net positive fluid balance and contribute toward daily total water intake.
- How much extra should I drink when exercising?
- This calculator adds 5–20 ml per kg of body weight depending on training duration. ACSM also suggests 400–600 ml in the 2 hours before exercise and 150–350 ml every 15–20 minutes during exercise as a practical pacing rule.
- Can I drink too much water?
- Yes — overdrinking (hyponatremia) is possible, especially during endurance events. If you produce pale-yellow urine every few hours, you are usually well hydrated. Drink to thirst plus the planned exercise add-on rather than forcing extra fluid.
- Why does climate change my target?
- Hot or humid conditions raise sweat rate dramatically. We add 350 ml for warm weather (24–30 °C) and 700 ml for hot weather (> 30 °C) to roughly replace the additional sweat losses for a sedentary day.