During a marathon, you should drink a mix of water and a sports drink containing electrolytes and carbohydrates, aiming for roughly 400 to 800 milliliters (about 14 to 27 ounces) per hour. The exact amount depends on your sweat rate, body size, pace, and the weather. Getting this right matters more than most runners realize: drinking too little hurts performance, but drinking too much can cause a dangerous condition called hyponatremia.
How Much Fluid Per Hour
There is no single number that works for everyone. A larger runner pushing seven-minute miles in 80-degree heat might need close to 2 liters per hour, while a smaller runner at a 10-minute pace in mild conditions could get by on half a liter. The safe and effective range for most marathon runners falls between 400 and 800 milliliters per hour, which works out to roughly 14 to 27 ounces.
A practical approach is to take three or four long sips every 15 minutes rather than chugging a full cup at each aid station. This keeps absorption steady and reduces the sloshing, bloated feeling that comes from drinking too much at once. If you’re racing in cool weather and running at a comfortable pace, stay toward the lower end of that range. If it’s hot or you’re a heavy sweater, push closer to the upper end.
Water, Sports Drinks, or Both
Plain water replaces fluid but nothing else. During a marathon, you lose both water and sodium through sweat, and you burn through your stored carbohydrates. A sports drink addresses all three at once. Research shows that drinks with a 6 to 8 percent carbohydrate concentration, the standard in most commercial products like Gatorade and Powerade, help runners maintain pace better than water alone. Drinks with higher sugar concentrations (around 10 percent) tend to sit in the stomach too long, slowing absorption and increasing the chance of nausea.
If you want the best hydration effect, look for hypotonic sports drinks, which have a slightly lower concentration of dissolved particles than your blood. A meta-analysis in Sports Medicine found that hypotonic drinks were superior to isotonic, hypertonic, and plain water for maintaining blood volume during continuous exercise. Isotonic drinks, the category most mainstream sports drinks fall into, actually performed worst for hydration at higher exercise intensities. That said, isotonic drinks still deliver carbohydrates and sodium effectively. Many runners alternate between water and a sports drink at aid stations, which is a reasonable middle ground.
Sodium and Electrolyte Needs
The American College of Sports Medicine recommends 300 to 600 milligrams of sodium per hour during prolonged exercise. Most standard sports drinks contain some sodium, but not always enough for heavy sweaters or hot-weather races. A typical 12-ounce serving of Gatorade, for example, has around 160 milligrams of sodium, so you may need to supplement with salt tablets or electrolyte capsules to hit that 300 to 600 milligram target.
Sodium matters because it helps your body absorb and retain the water you drink. Without adequate sodium, fluid passes through you quickly and your blood sodium levels can drop. Runners who drink large volumes of plain water without replacing sodium are the ones most at risk for hyponatremia.
The Danger of Drinking Too Much
Hyponatremia, a condition where blood sodium drops to dangerously low levels, is a real risk in marathon running. It happens when you drink more fluid than your body can process, diluting the sodium in your bloodstream. Symptoms range from nausea and confusion to seizures in severe cases. Research on marathon runners has identified several risk factors: drinking three or more liters total during the race, being female, having a BMI under 20, and running at a slower pace (which means more time on the course and more opportunities to drink).
The key rule is simple: don’t drink more than about 32 ounces (roughly one liter) per hour, and never force yourself to drink beyond thirst. Gaining weight during a marathon is a red flag that you’re overhydrating. Your goal is to prevent excessive dehydration, defined as losing more than 2 percent of your body weight, not to replace every drop of sweat in real time.
Adjusting for Hot Weather
Heat and humidity change the equation significantly. Your sweat rate climbs, so you need more fluid, but the ceiling on safe intake doesn’t move much. The solution is to favor sports drinks over water so that every sip delivers electrolytes alongside fluid. In hot conditions, drinking 5 to 8 ounces of a sports drink every 20 minutes is a reasonable target. Starting your pre-race hydration early helps too: 16 ounces of sports drink about an hour before the start tops off your fluid stores without overloading your stomach.
Pay attention to the heat index, not just the temperature. High humidity prevents sweat from evaporating, so your body produces even more sweat to compensate. If you haven’t trained in similar conditions, scale back your pace goal and be more deliberate about drinking at every aid station rather than skipping them.
How to Calculate Your Sweat Rate
The most useful thing you can do before race day is figure out your personal sweat rate during a training run. The process takes one run and a bathroom scale:
- Weigh yourself naked before the run in kilograms for easy math.
- Run for one hour at your expected marathon pace in conditions similar to race day.
- Track every ounce you drink during the run.
- Weigh yourself naked again immediately after.
Your sweat rate equals your pre-run weight minus your post-run weight, plus whatever fluid you drank during the run. If you weighed 60 kilograms before, 59 kilograms after, and drank nothing, you lost one liter per hour. That one liter is your target to replace, or at least get close to, every hour on race day. Subtract any bathroom stops from the calculation if relevant. Run this test two or three times in different temperatures to get a range you can work with.
Aid Stations vs. Carrying Your Own
Most marathons place aid stations every 1 to 2 miles, offering water and sometimes a sports drink. For the majority of runners, this is frequent enough to stay hydrated without carrying anything. Running without a vest or bottles saves weight, reduces heat buildup, and feels noticeably lighter, which adds up over 26.2 miles.
There are situations where carrying your own supply makes sense. If you use a specific sports drink or electrolyte mix that won’t be on the course, a small handheld bottle or belt lets you control exactly what you’re taking in. Hot-weather races are another case: aid stations occasionally run low on fluids for back-of-pack runners, and having your own water removes that worry. Runners who carry hydration also have the advantage of sipping on their own schedule rather than waiting for the next station.
Whatever you choose, practice it in training. Race day is not the time to try a hydration vest for the first time or to experiment with a sports drink brand your stomach hasn’t tested. Your gut is under stress during a marathon, and unfamiliar fluids are a common cause of nausea and cramping in the later miles.
What to Drink After You Finish
Once you cross the finish line, your goal shifts to replacing whatever fluid deficit remains. Weigh yourself after the race if you can: the difference between your pre-race and post-race weight tells you how much fluid you still need. Replace it gradually over the next few hours rather than all at once. A sports drink or something with sodium helps your body hold onto the fluid instead of sending it straight to your bladder. Pairing your drinks with salty foods like pretzels or broth works just as well if you’re tired of sweet beverages.