Fluid needs vary a lot by climate, sweat rate, and activity, so any calculator here is a starting baseline rather than a precise prescription. This one scales a standard per-kilogram estimate up for time spent exercising.
The baseline uses about 35ml of water per kilogram of body weight per day, then adds roughly 350ml for every 30 minutes of exercise to cover extra fluid lost through sweat. Hot or humid climates, and higher-intensity training, both push real needs above this estimate.