Hydration for runners: from 5K to marathon

2026-10-30

For runs under about an hour, including most 5K and 10K races, you don't need to drink during the run if you started well hydrated. For a half marathon, a few sips to thirst are usually enough. For a marathon and beyond, drink to thirst, typically in the range of about 0.3–0.8 liters (10–27 fl oz) per hour, add sodium and carbohydrates, and above all don't finish heavier than you started: in long races, drinking too much is a bigger health risk than drinking too little.

Drink to thirst or drink to a plan?

This used to be a heated debate. Older advice told runners to drink "before you're thirsty" and to replace all sweat losses. That changed after a series of hyponatremia cases (dangerously low blood sodium) in marathons, almost always in runners who drank too much.

In a study of the 2002 Boston Marathon, 13% of the 488 runners tested at the finish had hyponatremia, and 0.6% had critical levels. The strongest risk factors were weight gain during the race, a finish time over 4 hours, and very low or high body mass index; drinking more than 3 liters during the race and drinking at every mile were also linked. Notably, the type of drink (water or sports drink) made no difference.

An international consensus statement on exercise-associated hyponatremia (Hew-Butler et al., 2015) concluded that drinking when thirsty is the safest individual strategy. Thirst prevents most people from overdrinking while still protecting them from significant dehydration.

That doesn't mean plans are useless. A rough plan based on your own sweat rate is helpful for knowing how much to carry and where to drink. Use thirst as the steering wheel and the plan as a map.

Before the run

Drink normally the day before. There's no benefit to forcing extra liters; it just means more toilet stops.
2–4 hours before a long run or race: about 5–10 ml per kg of body weight, roughly 12–24 fl oz (350–700 ml) for a 155 lb (70 kg) runner, according to the German Nutrition Society's position on fluid replacement.
Last 30 minutes: a few sips if you're thirsty, then a last toilet visit.
Light yellow urine before the start is a good sign.

By distance

5K and parkrun

You don't need to drink during the race. A glass of water in the hour before and a drink afterwards are enough, even on warm mornings. Skipping the water station costs you nothing.

10K

Most runners finish in under an hour and don't need to drink. On a hot day or if you'll be out longer than about an hour, take a few sips at a water station when you're thirsty.

Half marathon

Here drinking starts to make sense, especially for runners finishing in 1:45 or later, or in warm weather. Sips at some water stations, guided by thirst, usually cover it. A gel or sports drink in the second half can help your energy, but it isn't essential for every runner.

Marathon

Fluids: drink to thirst. The German position paper cites thirst-guided athletes drinking about 300–600 ml (10–20 fl oz) per hour in 3–6 hour events, and the American College of Sports Medicine considers about 0.4–0.8 liters per hour reasonable for endurance exercise. Smaller, slower runners in cool weather will be at the low end; larger, faster runners in the heat at the high end.
Carbohydrates: for efforts over about 90 minutes, about 30–60 g of carbohydrate per hour supports performance. That's roughly one to two gels per hour, or a sports drink.
Sodium: worth adding if you sweat heavily (over about 1.2 liters per hour) and are running for more than about 2 hours. Sports drinks, salted gels or electrolyte tablets can provide it. See electrolytes: when do you need them?
Don't drink at every station by default. If you're running for 4–5 hours and drinking a cup at every station, you can easily drink more than you sweat.

Ultras and trail running

The same principles apply over a longer time: drink to thirst, take in sodium and carbohydrate regularly, and eat real food. Because ultras last many hours, even small extra drinks at every aid station add up, so the overdrinking risk is real. Weighing yourself at the start and checking at medical stations, if offered, is a good safeguard.

The scale test: are you drinking the right amount?

Weigh yourself before and after your long training runs:

You lost 1–2% of your body weight: normal and fine.
You lost more than about 2–3%: drink a little more next time, especially in the heat.
You weigh the same or more than at the start: you drank too much. Cut back.

The sweat rate guide shows the full method with a worked example.

Hot days

Heat raises your sweat rate and the strain on your body. Slow down your pace, start earlier in the day, wear light clothing, and expect to drink more than usual. On race day in the heat, it's a good idea to pour water over your head and neck to cool down; it helps without adding fluid to your stomach. More in how to stay hydrated in hot weather.

Warning signs during a race

Stop and seek help at a medical station if you have:

a headache, nausea or vomiting, confusion, or unusual bloating and swollen hands, especially if you've been drinking a lot. These can be signs of hyponatremia, which the Mayo Clinic lists among the risks of drinking too much water in marathons and triathlons. Don't drink more water to "fix" it.
dizziness, hot skin that has stopped sweating, or confusion: possible heat illness.

Practise in training

Practice drinking from cups while running (squeeze the top to make a spout).
Test your gels and sports drink on long runs; never try a new product on race day.
Check which drinks are offered on the course and where the stations are.
Decide whether you'll carry a bottle, a soft flask or a hydration vest.

After the run

Have a drink and a meal or snack with some salt and carbohydrates. If you have a race or hard session again within a day, aim for about 1.5 liters per kilogram lost (about 23 fl oz per pound) over the next few hours. Otherwise, normal eating and drinking will rehydrate you. For more on recovery see how much to drink before, during and after exercise.

Between runs, Water Tracker helps with the easy part: your everyday intake. Set yourself as active and the suggested goal adds 500 ml, and on the Apple Watch you can log a drink with the Digital Crown right after your cool-down, even if you left your iPhone at home.

The bottom line

5K and 10K: no need to drink during the race in most cases.
Half marathon: sips to thirst, more in the heat.
Marathon and beyond: drink to thirst (roughly 0.3–0.8 l per hour), add carbohydrates and, for heavy sweaters, sodium.
Never finish heavier than you started; weight gain is the clearest sign of overdrinking.
Practise your drinking and fuelling in training, not on race day.

This article is general information, not medical advice. If you have a health condition, are pregnant or take medication, ask your doctor how much you should drink.