You are running a 10 km race and wondering whether you should bring a bottle. You are preparing for your first 21 km and do not know what to put in it. The truth is that hydration while running does not work the same way over 30 minutes as it does over three hours. Here are the practical guidelines, distance by distance, so you can stop running by feel.
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 reaches the finish with a serious deficit.
But sweat is not just water. It also carries away minerals, especially sodium. Studies put losses between 800 and 1600 mg of sodium 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 are probably a heavy salt loser.
What dehydration does to your performance
A loss of water equivalent to 2% of your body weight is enough to reduce your performance. Specifically, for a 70 kg runner, that amounts to 1.4 liters. Your heart beats faster for the same effort, your temperature rises, your stride deteriorates, and your concentration falls apart toward the end.
The signs to watch for: marked thirst, dry mouth, legs that feel heavy for no reason, an abnormally high heart rate, headaches, and dark urine after the run. None of these signs is alarming on its own, but together they mean you have fallen behind.
How much to drink according to the distance
5 and 10 km: your thirst is almost always enough
For efforts under an hour and in moderate temperatures, you do not need to drink during the race. Your body handles a short effort very well without fluid intake. What matters is reaching the start 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 a water station make a difference toward the end of the race. And if you run at noon in midsummer, take a bottle even for 8 km.
The half marathon, 21 km: strategy becomes useful
Between 1 hour 20 minutes and 2 hours 30 minutes of effort, you enter the zone where hydration clearly affects your finishing 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 all 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 meets 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 your blood sodium 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. The calculation then depends on the distance between two water points, not just on time. Always allow 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 and 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 longer, when you need to drink and fuel at the same time.
Electrolytes alone
Sodium, potassium and magnesium with no added sugar. Useful when you are 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 on 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 conventional sodas. If you are looking for this type of format, Buddy hydration drinks provide 252 mg of sodium, 300 mg of potassium and 56 mg of magnesium per bottle.
Before, during, after: your three-stage plan
Before the outing
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
Take small, regular sips—never large amounts at once. Set yourself a schedule, for example at every aid station or every 15 minutes, rather than waiting until you feel thirsty. During long efforts, thirst often arrives when you are already behind.
After the run
Aim for about 1.5 times the lost volume, spread over the following four to six hours. The most reliable way to know: weigh yourself before and after a long outing. Each kilogram lost corresponds to one litre to replace. Add sodium to this rehydration, otherwise some of what you drink will go straight back out in your urine.
The three costliest mistakes
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 blood sodium levels. The symptoms resemble those of dehydration: headaches, nausea, confusion, and bloating. In long events, this situation is more dangerous than the opposite deficit.
Trying a new drink on race day
Your stomach needs to learn. A drink that is too sugary or too concentrated can trigger digestive problems during a race. Test everything in training, over at least two or three long runs, before settling on your strategy.
Forgetting about non-running days
Your hydration doesn’t start during your warm-up. A runner who is chronically dehydrated during the week starts Sunday morning with a deficit. The goal is simple: hydrate regularly every day, with a little extra on intense training days and during heatwaves.
FAQ: Your questions about hydration while running
Should I drink during a 10K?
At normal temperatures and for an effort lasting less than an hour, this isn’t 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 fluid 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 does the job very well.
How can I tell if I lose a lot of salt when I 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 for efforts lasting more than 90 minutes, in hot weather, or for runners who lose a lot of salt. On a 45-minute run in autumn, they provide no measurable benefit.
What to remember
For runs under an hour, water and your thirst are enough. Between 1 and 3 hours, aim for 400 to 600 ml per hour with sodium intake. Beyond 3 hours, sodium becomes essential and carbohydrates enter the equation. After exercise, allow 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 sweat-loss rate and can finally adjust your strategy to your body rather than to an average.
