Drink too little during exercise and you slow down, get a headache, and feel worse than the workout deserves. Drink far too much and you can feel bloated or unwell in a different way. The right amount sits between those extremes, and it depends on three things: how long you exercise, how hard you work, and how much you weigh. This calculator turns those three inputs into a drinking rate per hour and a total for the session, so you can plan bottles instead of guessing.
The model uses average sweat rates for a 70-kilogram person: light exercise loses about 0.4 litres per hour, moderate exercise about 0.7, and intense exercise about 1.0. Your own rate scales with weight, so a 90-kilogram athlete at moderate intensity loses about 0.9 litres per hour, while a 55-kilogram exerciser loses about 0.55. Multiply the hourly rate by the workout length in hours, and you get the total fluid to replace. A 70-kilogram person training moderately for one hour therefore needs about 700 millilitres.
Here is a worked example with different numbers. An 80-kilogram runner trains intensely for 90 minutes. The hourly rate is 1.0 times 80 divided by 70, which is about 1.14 litres per hour. Over 1.5 hours that is about 1.71 litres in total. That runner should plan roughly two standard bottles and sip through the session, rather than carrying one bottle and hoping. For a 60-minute moderate session at 70 kilograms, the answer is simpler: about 700 millilitres, or a little under one large bottle.
Heat and humidity change everything, and the calculator is honest about that. A moderate session in cool weather and a moderate session in summer heat can differ by half a litre per hour or more, because sweat exists to cool you, and hotter air demands more cooling. Clothing matters too: heavy kit, a helmet, or a non-breathable jacket traps heat and raises sweat. If you train in heat, treat the calculator result as a minimum and drink more, watching for the signs of dehydration like a dry mouth, dark urine, and a headache that grows during the session.
How you drink matters almost as much as how much. Small, regular sips beat rare, large gulps, because your stomach absorbs fluid gradually and large volumes slosh uncomfortably during movement. Start the session already hydrated, sip every fifteen to twenty minutes, and keep drinking after you finish, because you keep sweating while you cool down. For sessions under an hour in mild weather, plain water is usually enough. For longer or hotter sessions, food and sports drinks that replace salts become worth considering, since sweat loses salt as well as water.
Drinking too much is a real risk that deserves a plain warning. Overdrinking during long endurance events can dilute the salt in your blood, a condition that causes confusion, nausea, and swelling, and can become dangerous. The safest guide alongside any plan is thirst: drink when you are thirsty, use the calculator as a rough target, and do not force fluids far beyond thirst for hours on end. Weighing yourself before and after a session is a practical check: each kilogram lost is roughly one litre of fluid to replace afterwards.
Use this calculator when you plan a session, a run, a ride, or a match, and pack water to match the total it gives. Note the weather, your weight, and how you felt, and adjust next time: if you finished thirsty with a headache, the estimate was low for you; if you finished bloated, it was high. Over a few sessions you will learn your own sweat rate, which is far more useful than any average. Hydration is not complicated, but it is personal, and a planned bottle beats a guessed one every time.