Estimate daily fluid needs from body weight, training time and heat
Start with 300–500 ml after waking, spread the rest between meals, and add 200–300 ml before and after training — do not wait until you are thirsty.
Roughly 35 ml per kilogram of body weight, plus 12 ml per minute of exercise (about 0.7 L of sweat per hour), with another 10% in hot weather. Heavy sweaters and dark urine need more. If you have heart failure, kidney disease or a fluid restriction, follow your doctor instead — these numbers do not apply.