A marathon runner in active training typically needs about 0.5 to 1 ounce of water per pound of body weight each day, with additional fluid to replace what’s lost during runs. For a 150-pound runner, that works out to roughly 75 to 150 ounces daily, depending on training intensity, climate, and individual sweat rate. But the total number matters less than learning your own body’s signals and losses, because drinking too much can be just as dangerous as drinking too little.
Your Sweat Rate Sets Your Real Number
General guidelines are a starting point, but the most useful thing a marathon runner can do is calculate their personal sweat rate. The CDC formula is straightforward: weigh yourself before a run, weigh yourself after, add back the weight of any fluid you drank during the run, subtract urine volume, and divide by the number of hours you exercised. The result is your hourly fluid loss.
For example, if you weigh 155 pounds before a one-hour training run, drink 16 ounces during it, and weigh 153.5 pounds afterward, you lost roughly 40 ounces of fluid in that hour (24 ounces from weight loss plus the 16 you replaced). Do this test a few times in different weather conditions and you’ll have a reliable personal range. Most runners lose somewhere between 16 and 64 ounces per hour, and the variation is enormous. Heavier, faster runners in warm conditions sweat far more than lighter, slower runners in cool weather.
Once you know your sweat rate, your daily water goal during training becomes clearer: your baseline intake (roughly half your body weight in ounces) plus whatever you need to fully replace training losses. A good check is urine color. Pale yellow means you’re well hydrated. Clear and copious means you may be overdoing it.
How Much to Drink During Runs
The International Marathon Medical Directors Association recommends drinking when you’re thirsty, not on a fixed schedule, and capping intake at 400 to 800 milliliters per hour (roughly 13 to 27 ounces). Faster, heavier runners competing in heat should aim toward the higher end. Slower runners and walkers in cooler conditions should stay toward the lower end. The ACSM similarly advises no more than one quart (32 ounces) per hour during exercise.
Multiple studies have found that drinking when thirsty performs just as well as aggressive scheduled drinking. The “drink as much as possible” approach that was popular for decades has fallen out of favor because it increases the risk of a potentially life-threatening condition called hyponatremia, where blood sodium drops dangerously low.
Why Overdrinking Is a Real Danger
Hyponatremia happens when you take in more fluid than your body can excrete. During prolonged exercise, your body also releases a hormone that causes it to retain water rather than flush it out. Pain, emotional stress, nausea, heat exposure, and even common painkillers like ibuprofen can amplify this effect. The combination of drinking too much and retaining extra water dilutes your blood sodium to dangerous levels.
Mild cases cause bloating, nausea, and confusion. Severe cases can lead to seizures and are a medical emergency. The runners most at risk are slower marathoners and walkers who spend more time on the course and pass more aid stations, often drinking at each one out of habit rather than thirst. The single most effective prevention strategy is simple: drink to thirst, not beyond it.
Electrolytes and Sodium Needs
Water alone doesn’t cover your losses during marathon training. Sweat contains sodium, and replacing fluid without replacing sodium is what drives hyponatremia risk. The general population recommendation is about 1,500 milligrams of sodium per day, but endurance athletes need significantly more on training days.
During prolonged runs, the ACSM recommends 300 to 600 milligrams of sodium per hour. You can get this from a sports drink containing 230 to 690 milligrams of sodium per liter, which is the range found in most commercial sports drinks and allows for optimal fluid absorption. Some runners prefer salted snacks or electrolyte tablets alongside water, which also works. Sodium concentrations above 1,000 milligrams per liter tend to taste unpleasant and aren’t necessary for most people.
On non-training days or easy days, standard dietary sodium from meals is usually sufficient. On long run days, especially in heat when sweat rates climb, actively supplementing sodium becomes more important. When your sweat rate is elevated, aiming for 300 to 600 milligrams of sodium per hour of exercise helps maintain fluid balance and stimulates the thirst mechanism that keeps you drinking at an appropriate rate.
The 48-Hour Pre-Race Window
Hydration for race day doesn’t start on race morning. UCSF Health recommends making a conscious effort to stay well hydrated for at least 48 hours before your marathon. This doesn’t mean guzzling water. It means consistently sipping throughout the day and monitoring your urine color so you arrive at the start line already in good shape.
On race morning, drink two 8-ounce glasses of water or a sports drink about two hours before the gun goes off. This gives your body time to absorb the fluid and lets you use the bathroom before you start. If you expect to take longer than five hours to finish, your sweat losses will generally be lower, so you won’t need as much pre-race fluid.
Putting It All Together
Daily hydration for marathon runners breaks down into three layers. First, your baseline: roughly half your body weight in ounces spread throughout the day. Second, your training replacement: enough fluid during and after runs to match your measured sweat losses, staying within the 400 to 800 milliliter per hour range during exercise. Third, your electrolytes: 300 to 600 milligrams of sodium per hour of sustained effort, plus normal dietary sodium the rest of the day.
The most important principle across all three layers is the same one: let thirst guide you. Your body has a finely tuned system for signaling when it needs fluid. Runners who respect that signal, rather than overriding it with aggressive drinking schedules, tend to perform better and avoid the serious risks that come with overhydration. Test your sweat rate in training, find the intake level that keeps your urine pale yellow without making you feel sloshy, and trust that number on race day.