How much should you drink during a marathon?

Enough to limit body-mass loss to a few percent, which for most runners means drinking to thirst. Sweat rates range roughly 0.8–1.5 L/hr and vary enormously between individuals.

Sweat rates in running typically fall between 0.8 and 1.5 litres per hour, but the spread between individuals is far larger than that range suggests — heat, humidity, body size, pace and acclimation status all shift it. There is no universal millilitres-per-hour prescription, and any advice that offers one is ignoring most of the variance.

The evidence-based target is to limit body-mass loss over the race rather than to hit a drinking volume. Losses beyond roughly 2–3% of body mass begin to impair performance, particularly in heat, through the plasma-volume and cardiac-drift mechanism. Losses under that are generally well tolerated.

For most runners in most conditions, drinking to thirst achieves that without any arithmetic. Thirst is an imperfect but reasonable regulator over marathon durations, and it has the significant advantage of not systematically overshooting — which is the failure mode that causes real harm.

To know your own numbers, weigh yourself before and after a long run in race-like conditions, accounting for what you drank. That gives a sweat rate you can plan around, and it is the only way to find out whether you are an unusually heavy or light sweater.

Glossary: Hyponatremia, Cardiac Drift