How much water should I drink during a marathon?

Aim to drink to thirst rather than on a rigid schedule, which aligns intake with actual fluid losses. For most runners, that works out to roughly 400 to 800 mL per hour, with the lower end in cool conditions and the upper end in heat. Drinking too little risks dehydration and performance decline; drinking too much risks hyponatremia, a dangerous dilution of blood sodium. Weigh yourself before and after a long training run to measure your personal sweat rate, then use that number to set a sensible target range for race day rather than relying on generic advice.