Half Marathon Hydration: How Much to Drink and When
Dehydration starts costing you performance at 2% of body weight lost. Here is the practical way to hydrate before, during and after your 21K.
Dehydration starts costing you performance at 2% of body weight lost. Here is the practical way to hydrate before, during and after your 21K.
Hydration is one of the biggest levers on half marathon performance, and one of the most improvised. I have seen perfectly trained runners give away several minutes in the closing kilometres simply because they did not drink enough — or because they drank too much of the wrong thing. This guide covers everything you need to reach the finish line properly hydrated.
-2%
-2% body water = -5% performance
400-800ml/h
Sweat produced per hour depending on temperature and intensity
Most runners
Most runners drink too little, not too much. Thirst arrives late
Clear urine
The most reliable hydration test: clear urine = well hydrated
The human body is roughly 60% water. During hard exercise that balance shifts quickly through sweat. In normal temperatures a runner loses between 400 and 800ml of fluid per hour. In hot or humid conditions that figure can go beyond 1,200ml per hour.
The direct consequence of not replacing that fluid is a drop in blood volume, which forces the heart to work harder to move oxygen to the muscles. Once fluid loss passes 2% of body weight — only 1.4 litres for a 70 kg runner — performance falls by 5 to 10%. In a half marathon that can mean 5-10 extra minutes.
The good news is that a hydration strategy is fairly easy to execute well. You just have to plan it before the race instead of improvising at the aid stations.
Race hydration starts long before race day. In the 24-48 hours beforehand, the goal is simply to reach the start line in an optimal fluid state.
Race-day hydration follows its own logic. You want to reach the start line well hydrated, but not with a stomach full of water.
Most half marathons have aid stations every 5 km. A solid strategy means drinking at every single one of them, whether you feel thirsty or not. Thirst shows up once dehydration has already started: it is a late signal, not an early one.
| Temperature | Estimated loss/hour | What to drink at each station |
|---|---|---|
| Below 10°C | 300-500ml/hour | Water (100-150ml per station) |
| 10-18°C | 500-700ml/hour | Alternate water and sports drink |
| 18-25°C | 700-900ml/hour | Mainly sports drink (150-200ml per station) |
| Above 25°C | 900-1200ml/hour | Sports drink at every station (200ml) + extra water if available |
Rule 1: Drink at every aid station
Do not wait until you are thirsty. Do not skip a water point because you "feel fine". Dehydration builds gradually and performance drops before the brain sends the thirst signal.
Rule 2: Walk for 8-10 seconds and drink properly
Trying to drink at race pace means half of it ends up on your shirt and you swallow more air than you should. Walking 8-10 seconds to drink properly works out at less than 15 seconds of total loss. A small price for hydration that actually lands.
Rule 3: The right amount, neither too little nor too much
The ideal amount at each aid station is between 100 and 200ml. More than 300ml in one go can leave you feeling heavy and uncomfortable while running. Less than 100ml is not enough to cover what you have lost since the last station.
This is one of the most common questions, and the answer depends mainly on how long your race lasts and on the temperature outside.
Water only if:
Alternate water and sports drink if:
Prioritise sports drink if:
If you want to back up your race hydration with a good sports drink in training, take a look at our comparison of the best hydration drinks for running, where we break down electrolytes, osmolality and taste for each option available in Spain.
Hyponatremia — low blood sodium caused by taking on too much water without electrolytes — is a real problem for slower runners who finish a half marathon in over 2h30 and drink large amounts of plain water along the way.
Rehydration starts the moment you cross the line. Your body has lost fluid, electrolytes and glycogen, and the first 60 minutes after the race are the most important window for recovery.
Weather conditions are the factor that changes your hydration strategy the most. The same runner can need twice as much fluid in a 25°C race as in a 10°C one.
Cold — below 10°C
Estimated loss: 300-500ml/hour. Dehydration risk gets underestimated in the cold because you simply feel less thirsty.
Cool to mild — 10-22°C
Estimated loss: 500-900ml/hour. The ideal conditions for most half marathons. Go with a balanced strategy.
Heat — above 22°C
Estimated loss: 900-1,200ml/hour. Conditions that call for adjusting your pace and maximising hydration.
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Should I walk through the aid stations to drink?
Yes, in most cases. Walking for 8-10 seconds so you can actually drink costs you less than 15 seconds of total time, and it stops you choking or spilling the water. Hydration that works pays that time back several times over.
How much water do I lose in a half marathon?
It depends on the heat and the intensity. In cool conditions (10-15°C): 600-900ml per hour. In the heat (above 22°C): up to 1,200ml per hour. A 2-hour half marathon in hot weather can mean a total loss of 2-2.4 litres.
Can I drink only sports drink during the race?
Yes, and it is even better than water alone if the race lasts more than 90 minutes, because it replaces the electrolytes you lose through sweat. A sports drink also lowers the risk of hyponatremia on long races.
Should I carry my own bottle or drink at the aid stations?
It depends on the race. For half marathons with aid stations every 5km, the water points are enough. If it is very hot or you are running slower than 2h30, consider carrying a small bottle or a compact hydration vest.
Is it true that drinking too much water is dangerous?
Yes. Hyponatremia (too much water without electrolytes) can be dangerous in slower runners who drink only water for more than 2-3 hours. Alternate water with a sports drink to keep your sodium balance.
How long before the race should I stop drinking?
Stop drinking 20-30 minutes before the gun. If you drink right before the start, you will run the first few kilometres with a full bladder, which is uncomfortable and may force a toilet stop.
What if nerves stop me drinking the day before?
That is normal. Sip small amounts through the day. The most reliable tool is the colour of your urine: if it is clear or pale yellow, you are well hydrated. Do not force large volumes if you are not thirsty.
Do caffeinated drinks (coffee, cola) count towards hydration?
Coffee in moderate doses (1-2 cups) does not dehydrate you significantly according to current evidence. But do not swap water for coffee the day before the race. Sugary drinks like cola are not a good pre-race option.
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