Water Intake Calculator
A daily water target from your body weight and training time.
The estimate
35 ml per kg of body weight, plus ~350 ml per 30 minutes of exercise
For a 70 kg person training half an hour, that's about 2.8 liters a day. Roughly 20% of fluid normally arrives via food, and coffee and tea count toward the total; the caffeine diuresis myth doesn't survive measurement at normal intakes.
Thirst is a decent gauge
Healthy kidneys tolerate a wide range, so treat the target as a reference, not a rule. Pale-yellow urine means you're fine. Drink more in heat, at altitude, when pregnant or breastfeeding, and during illness; drink to thirst during endurance events rather than forcing fluid, since overdrinking (hyponatremia) is the more dangerous mistake in marathons.