During a marathon, most runners need 400–800 ml of fluid per hour, adjusted to sweat rate and weather. Aim to finish 1–2% lighter than you started — drinking to zero weight loss increases the risk of hyponatraemia.
Marathon hydration is a balance between two failures. Drink too little and performance falls, core temperature rises, and the last 10 km become brutal. Drink too much and you risk exercise-associated hyponatraemia — which, in marathon medical tents, is a more common serious problem than dehydration. The target zone for most runners is 400–800 ml per hour.
Individual sweat rates range from about 0.5 to over 2.0 litres per hour, which is why generic advice is so unreliable. Test it once:
Replace roughly 60–80% of that hourly rate during the race. Full replacement is rarely achievable and rarely necessary.
Over three to five hours you can lose 2–7 grams of sodium in sweat. Replacing that fluid with plain water alone is what drives hyponatraemia. Use a sports drink providing roughly 300–700 mg of sodium per litre for the bulk of your in-race fluid, and keep plain water for rinsing or cooling.
Replace roughly 1.25–1.5 litres for every kilogram of body weight lost, spread over the following 4–6 hours, with sodium included through food or drink. Drinking it all in the first 30 minutes mostly produces urine rather than rehydration.
400–800 ml per hour covers most runners. Lighter runners in cool weather sit near the bottom of that range; heavy sweaters in heat can exceed it. The reliable way to personalise it is a sweat-rate test in training.
Not automatically. Aid stations are typically 2–3 km apart, which can mean 8–12 stops. Drinking a full cup at each one often exceeds what you need. Drink to thirst and to your planned hourly volume instead.
Weigh yourself naked before and after a one-hour run at race pace. Each kilogram lost equals about 1 litre of sweat. Add back anything you drank during the run to get your true hourly sweat rate.
It is dangerously low blood sodium, usually caused by drinking more fluid than you sweat during long events. Avoid it by not over-drinking, using sodium-containing sports drinks on longer efforts, and accepting a small weight loss over the race.