How much should you drink during a marathon?
Drink to thirst. In slower runners the main risk is drinking too much, which can dilute the blood's sodium to dangerous levels. At the Boston Marathon, finishing in over 4 hours and gaining weight during the race were the biggest risk factors.
What to do
- Drink when you are thirsty. Don't drink at every station just because it is there.
- If you want a number, about 400–800 mL an hour is a common starting range, and 400–600 mL if you expect to take more than 4 hours. These are consensus-based guides, not tested limits.
- Weigh yourself before and after a long run. If you are heavier afterwards, you drank too much.
- Don't count on salt tablets or salty drinks to protect you. They don't prevent the problem if you overdrink.
- Measure your sweat rate in training if you want a personal ceiling.
- Confusion, headache or vomiting after a race need medical help.
The real risk is too much, not too little
Many runners believe the danger in a marathon is dehydration. For slower runners, the research points the other way.
Drinking far more than you lose can dilute the sodium in your blood. This is called exercise-associated hyponatraemia: low blood sodium during or after exercise, usually from overdrinking. It can be dangerous.
At the Boston Marathon, researchers tested a sample of finishers (Almond and colleagues, 2005).
- 13% had hyponatraemia.
- Runners who finished in more than 4 hours had about 7.4 times the odds.
- Runners who gained weight during the race had about 4.2 times the odds.
Gaining weight during a marathon means you took in more fluid than you lost. Slower runners have more time and more drink stations to do that.
Many runners plan to overdrink
In a 2022 survey of marathoners, 60% of the slowest third planned to drink at every station. Fewer than 5% named drinking to thirst as a way to prevent hyponatraemia (Namineni and colleagues, 2022). The runners most at risk are often the ones with the riskiest plan.
Who should be most careful
This checklist of risk factors builds on the Boston and survey findings. Not every item was tested in those studies, so treat it as a practical guide. Take extra care with drinking if:
- You expect to finish in more than 4 hours.
- You are small, or your body weight is very low or very high for your height.
- It is your first marathon.
- You take anti-inflammatory painkillers (NSAIDs) or SSRI antidepressants.
- You plan to drink at every station.
- The race will be hot.
A simple check works in training too. Weigh yourself before and after your long runs. Any weight gain over a long run means you drank more than you needed. Treat it as a warning and drink less next time.
Drink to thirst
The international consensus on hyponatraemia recommends drinking to thirst (Hew-Butler and colleagues, 2015).
If you want a rough guide, coaching and app defaults based on that consensus suggest:
- 400–800 mL an hour if you haven’t measured your sweat rate.
- 400–600 mL an hour if you expect to finish in more than 4 hours.
- Never more than your measured sweat rate.
These ranges are conventions built on the consensus, not tested limits. Thirst comes first.
Sodium doesn’t protect you
Salt tablets and electrolyte drinks are often sold as protection. They aren’t. When athletes overdrink, sodium supplements do not prevent hyponatraemia (Hoffman and colleagues, 2015). The cause is too much fluid, and the fix is drinking less.
Sodium is optional in long or hot races or for very salty sweaters, typically 300–600 mg an hour (Klingert and colleagues, 2022). This is weaker evidence, and runners with high blood pressure, kidney or heart disease should ask their doctor first.
How much do runners actually sweat?
Endurance athletes lose on average about 1.28 litres of sweat and 52 mmol of sodium per hour, but the spread between people is wide (Barnes and colleagues, 2019). That is why a fixed drinking schedule suits some runners and badly overloads others.
Measuring your sweat rate
A sweat-rate test gives you a personal ceiling. This is the consensus method (Baker, 2017; Kenefick, 2018):
- Weigh yourself nude.
- Run 45–60 minutes at marathon pace, in weather like race day.
- Note how much you drink, and any urine passed.
- Weigh yourself nude again.
- Sweat rate per hour = (weight before − weight after in kg + fluid drunk in litres − urine in litres) ÷ hours run.
Repeat it on a cool day and a warm day. In the race, aim to lose no more than 2–3% of your body weight.
Take the right lesson from the fuelling trial
The Copenhagen Marathon trial, the only fuelling trial in recreational marathoners, gave runners a planned schedule of carbohydrate, caffeine and about 750 mL of water an hour. It was about 11 minutes faster than self-chosen fuelling (Hansen and colleagues, 2014). The lesson is about carbohydrate, not about drinking a fixed volume. Get your carbs from gels or a measured drink, and let thirst decide the rest.
Red flags
Confusion, headache or vomiting after a race means you should get medical help. Tell the medical team how much you drank during the race.
What we don’t know
- The Boston figures come from one race, published in 2005. Rates may differ by race and weather.
- The mL-per-hour ranges are conventions, not tested limits.
- Sweat rates vary widely, so averages say little about any one runner.
- The sodium guidance is weaker than the drinking guidance.
Sources
- Almond et al. (2005). Hyponatraemia among Boston Marathon finishers.
- Namineni et al. (2022). Survey of marathoners’ drinking plans and hyponatraemia knowledge.
- Hew-Butler et al. (2015). International consensus on exercise-associated hyponatraemia.
- Hoffman et al. (2015). Sodium supplementation and hyponatraemia in endurance athletes.
- Klingert et al. (2022). Sodium intake during endurance exercise.
- Barnes et al. (2019). Sweat and sodium losses in athletes.
- Baker (2017). Methods for measuring sweat rate and sweat sodium.
- Kenefick (2018). Planned drinking versus drinking to thirst.
- Hansen et al. (2014). Planned vs self-chosen fuelling in the Copenhagen Marathon.
Last reviewed 13 November 2026.