You're running a 10 km race and wondering whether you should bring a bottle. You're preparing for your first 21 km and don't know what to put in it. The truth is that hydration while running doesn't work the same way over 30 minutes as it does over three hours. Here are the practical guidelines, distance by distance, to stop running by feel alone.
Why hydration changes everything when you run
What you really lose when you sweat
When you run, your body gets rid of heat by producing sweat. Depending on the temperature, your build, and your intensity, you can lose between 0.5 and 2 liters per hour. A runner losing 1 liter per hour during a half marathon in full sun arrives at the finish with a serious deficit.
But sweat isn't just water. It also carries away minerals, especially sodium. Studies put sodium losses at between 800 and 1600 mg per liter of sweat, with major differences from one runner to another. If you find white marks on your cap or T-shirt after a long run, you're probably one of the heavier salt losers.
What dehydration does to your performance
A water loss equivalent to 2% of your body weight is enough to reduce your performance. In practical terms, for a 70 kg runner, that represents 1.4 liters. Your heart beats faster for the same effort, your temperature rises, your stride deteriorates, and your concentration goes haywire toward the end.
The signs to know: pronounced thirst, dry mouth, legs that seem heavy for no reason, an abnormally high heart rate, headaches, and dark urine after your run. None of these signs is alarming on its own, but together they mean you've fallen behind.
If you're interested in the minerals you lose beyond the race itself, our electrolyte drinks guide explains the mechanism in detail.
How much to drink depending on the distance
5 and 10 km: your thirst is almost always enough
For less than an hour of effort in moderate temperatures, you don't need to drink during the race. Your body handles a short effort very well without any intake. What matters is starting properly hydrated: 400 to 500 ml of water in the two hours beforehand, then a few sips 15 minutes before the starting gun.
The exception is heat. Above 25 degrees, even over 10 km, a few sips at an aid station make a difference toward the end of the race. And if you run at noon in the middle of summer, take a bottle even for 8 km.
The half marathon, 21 km: strategy starts to matter
Between 1 h 20 and 2 h 30 of effort, you enter the zone where hydration clearly affects your time. A reasonable target is between 400 and 600 ml per hour, in small, regular sips of 100 to 150 ml every 15 to 20 minutes. Drinking 500 ml at once at kilometer 12 is the best way to end up with a heavy stomach.
Over this distance, sodium intake starts to matter. A drink providing 400 to 700 mg of sodium per liter adequately covers the needs of a runner who sweats normally.
Marathon and trail running: sodium is no longer optional
After three hours of effort, water alone becomes a problem. You would dilute the sodium in your blood without ever replacing it. The recommended range rises to 500 or 800 ml per hour, with a consistent sodium intake, often supplemented with carbohydrates to maintain your energy.
For trail running, add altitude and sections without aid stations to the equation. You should then calculate based on the distance between two water points, not just on time. Always allow for a 20% margin.
Water, isotonic drink or electrolytes: what to put in your bottle
Plain water
Perfect for short outings, easy runs and sessions under an hour. Simple, free, easy to digest. Its only limitation appears during long efforts, when it replaces neither lost minerals nor consumed energy.
The isotonic drink
It contains water, electrolytes and carbohydrates at a concentration close to that of your blood, allowing for rapid absorption. This is the classic choice for races of 21 km and more, when you need to drink and fuel at the same time.
Electrolytes only
Sodium, potassium and magnesium with no added sugar. Useful when you're already managing your energy with gels or bars and only want to replace mineral losses. Many runners also appreciate this formula for rehydrating after a session or during hot days without training. Coconut water-based hydration drinks work well in this role because they combine a natural source of potassium with sodium and magnesium without the sugar load of classic sodas. If you're looking for this type of format, the Buddy electrolyte drink provides 300 mg of sodium, 300 mg of potassium and 56 mg of magnesium per bottle.
For long runs, a sugar-free formula prevents you from doubling up on the carbohydrates in your nutrition and those in your drink. This is the approach behind our powdered electrolytes, which can be adjusted according to the heat. Before choosing a brand, you can compare the actual sodium and sugar content of most drinks on the market at drinkscore.info.
Before, during, after: your three-stage plan
Before the run
Drink 400 to 600 ml in the two hours before, then stop 20 minutes before setting off to avoid running with a full bladder. A simple guideline: your urine should be clear or pale yellow. If it is dark before you even start, you are already at a disadvantage.
During the effort
Small, regular sips, never large amounts all at once. Set yourself a rhythm, for example at every aid station or every 15 minutes, rather than waiting until you are thirsty. During long efforts, thirst often arrives when you are already behind.
After the race
Aim for about 1.5 times the amount lost, spread over the following four to six hours. The most reliable way to know: weigh yourself before and after a long run. Every kilogram lost corresponds to one liter to replace. Add sodium to this rehydration, otherwise some of what you drink will pass straight out in your urine.
What happens just after the finish line matters just as much as what happens during the race. We go into more detail in our guide on what to drink after exercise.
The three mistakes that cost the most
Drinking far too much plain water
This is the classic trap for beginner marathoners who drink at every aid station as a precaution. Too much water without sodium can cause hyponatremia, in other words, a drop in sodium levels in the blood. The symptoms resemble those of dehydration: headaches, nausea, confusion, bloating. During long-distance events, this situation is more dangerous than the opposite imbalance.
Testing a new drink on race day
Your stomach needs to learn. A drink that is too sugary or too concentrated can trigger digestive issues in the middle of a run. Test everything in training, over at least two or three long runs, before settling on your strategy.
Forgetting the days without running
Your hydration does not begin at your warm-up. A runner who is chronically dehydrated during the week starts Sunday morning with a deficit. The goal remains simple: hydrate regularly every day, increasing it slightly on intense training days and during hot spells.
Regarding coconut water, often described as a natural drink for runners, we looked at what it really provides in our article on coconut water and sport.
FAQ: your questions about hydration while running
Should I drink during a 10K?
At normal temperatures and during an effort lasting less than an hour, this is not necessary if you are well hydrated at the start. Above 25 degrees or if you sweat a lot, a few sips halfway through are useful.
How long before running should I stop drinking?
About 20 minutes. This gives your body time to absorb the liquid and empty your bladder before the start.
Can coconut water replace an isotonic drink?
It is naturally rich in potassium, which makes it useful for rehydration. However, it contains little sodium compared with a runner's actual losses. During a long effort, a sodium-enriched drink remains more suitable. For recovery after a session, coconut water works very well.
How can I tell if I lose a lot of salt through sweat?
Two simple clues: white marks on your skin or clothes after a run, and sweat that stings your eyes. If both sound familiar, increase your sodium intake during efforts lasting more than an hour.
Are electrolyte tablets really useful?
They are genuinely useful during efforts lasting more than 90 minutes, in hot weather, or for runners who lose a lot of salt. During a 45-minute run in autumn, they provide no measurable benefit.
Key takeaways
For under an hour, water and your thirst are enough. Between 1 and 3 hours, aim for 400 to 600 ml per hour with a sodium intake. Beyond 3 hours, sodium becomes essential and carbohydrates enter the equation. After exercise, allow for 1.5 liters for every kilogram lost.
The key habit to adopt: weigh yourself before and after your next long run. With a single measurement, you'll know your personal rate of fluid loss and can finally adjust your strategy to your body rather than to an average.
