The calculator uses the widely accepted guideline of 30–40 ml per kilogram of body weight as a baseline. A 75 kg person needs roughly 2.25–3.0 L per day before accounting for exercise or heat. This baseline comes from EFSA (European Food Safety Authority) and NASEM (National Academies) recommendations.

Most athletes lose 400–800 ml of sweat per hour during moderate exercise. In hot conditions or at high intensities, losses can exceed 1.5 L/h. The calculator adds fluid based on your exercise duration and temperature. For a more precise number, use our Sweat Rate Calculator with actual pre/post-exercise weigh-ins.

Yes. While caffeine has a mild diuretic effect, the net fluid contribution from coffee and tea is positive. Studies show that habitual caffeine consumers retain most of the fluid from caffeinated beverages. They count toward your total, though plain water remains the most efficient hydrator.

Yes. Overhydration can lead to hyponatremia — dangerously low blood sodium levels. This is most common during long endurance events when athletes drink far more than they sweat. Aim to replace 75–80% of sweat losses rather than 100%, and include electrolytes during exercise lasting over 1 hour.

Heat increases sweat rate significantly. At temperatures above 30°C, your body produces more sweat to cool down, increasing fluid losses by 500 ml or more per hour. Humidity compounds this effect because sweat evaporates less efficiently, prompting even greater sweat production.