How much water should you drink during a 4 hour marathon?

December 31, 2025 •

5 min read

Losing just 2% of your body weight through sweat can significantly impair your marathon performance. A personalized approach, rather than a generic rule, is crucial when determining how much water should you drink during a 4 hour marathon to avoid both dehydration and dangerous overhydration.

Quick Summary

This guide outlines a personalized hydration strategy for a 4-hour marathon, including how to calculate your sweat rate and the critical role of electrolytes, focusing on a sip-and-replace approach to optimize performance and safety. It examines the factors influencing fluid needs and details a practical plan for race day, addressing common risks.

Key Points

Calculate Your Sweat Rate: Personalizing your hydration starts with a simple weigh-in test during training runs to determine your individual fluid loss per hour.
Electrolytes are Essential: Over a 4-hour race, electrolytes, especially sodium, are lost through sweat and must be replaced to prevent cramping and support bodily functions.
Avoid Overdrinking: Drinking too much plain water, especially for slower runners, can lead to hyponatremia, a dangerous condition caused by low blood sodium levels.
Sip, Don't Chug: Consuming fluids in small, regular amounts (e.g., every 15-20 minutes) is more effective and less likely to cause stomach distress than gulping large volumes.
Start Hydrated: Proper hydration begins days before the race, not just on race morning. Start the day well-hydrated by sipping fluids in the hours before the gun goes off.
Train with Your Plan: The best way to ensure your race day strategy works is to practice it repeatedly during your long training runs in similar conditions.

Your Personal Hydration Prescription

For a marathoner aiming for a 4-hour finish, relying on general advice can be a mistake. The optimal amount of fluid to consume varies dramatically based on your individual sweat rate, the race day temperature and humidity, your body size, and running intensity. A common ballpark figure is to aim for 300–600ml (10–20oz) of fluid per hour, but a tailored strategy is always superior. The goal is to replace a substantial portion of the fluid lost, without overcompensating and risking hyponatremia (water intoxication), a serious condition caused by low blood sodium. Practicing and fine-tuning your hydration strategy during long training runs is the most effective way to prepare for race day.

How to Determine Your Sweat Rate

Your sweat rate is the most important factor in determining your personal hydration needs. Measuring it during training runs in conditions similar to your race day is key. A simple sweat test involves weighing yourself before and after a run to measure fluid loss.

Weigh yourself nude before your run (note: an empty bladder is most accurate).
Drink a pre-measured amount of fluid during your run and log the volume.
After the run, towel off and weigh yourself again without clothes.
Subtract your post-run weight from your pre-run weight. The difference is your net fluid loss. (1kg = 1L of fluid lost; 1lb = ~16-24 oz of fluid lost).
Add the amount of fluid you consumed to your net fluid loss to get your total fluid loss. Divide this by your running duration to find your hourly sweat rate.

This calculation provides a clear, personalized target for your hourly fluid intake during the race, helping you maintain a body mass loss of under 2-3%.

The Crucial Role of Electrolytes

Electrolytes, particularly sodium, are minerals lost through sweat that play a vital role in maintaining the body's fluid balance and nerve function. Replacing these lost salts is essential to prevent muscle cramping, fatigue, and other performance issues. Relying solely on water during a marathon, especially in warm or humid conditions, can dilute your body's sodium levels and lead to the dangerous state of hyponatremia. Most marathoners, and particularly those completing a 4-hour marathon, will benefit from supplementing with electrolytes.

Dehydration vs. Overhydration

Both consuming too little and too much fluid can have serious consequences. While dehydration is a more common concern, overhydration poses a significant, and potentially life-threatening, risk.

Dehydration Symptoms: Dry mouth, fatigue, dizziness, muscle cramps, and dark yellow urine are common indicators. Inadequate fluid intake increases your core body temperature, decreasing efficiency and speed.
Overhydration (Hyponatremia) Symptoms: This is caused by diluted sodium levels and can lead to swelling in the extremities, headaches, confusion, nausea, and, in severe cases, seizures or comas. A sloshing stomach during a run is a telltale sign you’ve consumed too much fluid at once.

Hydration Strategy Comparison: Water vs. Sports Drinks

Choosing between plain water and a sports drink depends on individual needs and race duration. For a 4-hour marathon, sports drinks are generally recommended because they provide essential electrolytes and carbohydrates for energy.


Feature	Plain Water	Sports Drinks	Rationale for a 4-hour Marathon
Electrolytes	None	Yes (Sodium, Potassium, etc.)	Electrolytes are lost in sweat and crucial for maintaining fluid balance and preventing cramping over 4 hours.
Carbohydrates	None	Yes (e.g., glucose, fructose)	Sustained energy is needed over 4 hours; carbohydrates help fuel your muscles and prevent 'hitting the wall'.
Palatability	Neutral	Often flavored	The flavor of sports drinks can encourage consistent drinking, but can also cause flavor fatigue.
Risk of Hyponatremia	High (if overconsumed)	Low (when consumed correctly)	When taking on electrolytes, the risk is lower than with plain water, which can dilute blood sodium.
Optimal Use	Short, cool runs	Long, intense races (like a 4-hour marathon)	For endurance events, the combination of fluids, carbs, and electrolytes is most beneficial.

Your 4-Hour Marathon Hydration Plan

Based on your personalized sweat rate (aiming for 10-20oz per hour is a good start), here is a sample schedule for fluid intake during a 4-hour race. The goal is to consume small, regular amounts rather than large, infrequent gulps, which can cause stomach distress.

Before the Race

24-48 Hours Prior: Focus on consistent hydration throughout the day. Aim for pale yellow urine as a benchmark.
2-3 Hours Prior: Drink 16-20oz of water or a low-carb electrolyte drink. This gives your body time to absorb it and for you to use the restroom.
15-20 Minutes Prior: Sip another 6-8oz of fluid.

During the Race (per hour)

Miles 1-6 (Hour 1): Take small sips (e.g., 5-8oz) every 15-20 minutes, either from a handheld bottle or a hydration vest. Consider supplementing with a sports drink if using race aid stations later on.
Miles 7-13 (Hour 2): Continue sipping regularly. As intensity increases, so do sweat rates. Keep your total fluid intake for the hour within your personalized target. Start taking your planned energy gels or chews, remembering to wash them down with a small amount of water to help with absorption.
Miles 14-20 (Hour 3): Utilize the course's aid stations, alternating between water and sports drink. A good strategy is to grab two cups at each station—one for water and one for a sports drink.
Miles 21-26.2 (Hour 4): Maintain your routine of small, regular sips. As fatigue sets in, it's easy to forget to drink. Set a timer on your watch if needed.

Conclusion: Practice, Personalize, Perform

Proper hydration is a critical, yet highly personal, aspect of marathon success. While general guidelines exist, a cookie-cutter approach to how much water should you drink during a 4 hour marathon falls short. By understanding your individual sweat rate, recognizing the importance of electrolytes, and practicing your strategy on long training runs, you can create a race-day hydration plan that is both safe and performance-enhancing. Remember that consistent, small sips are better than large, infrequent gulps. Listen to your body and adjust your intake based on thirst and conditions, but always with a plan that prioritizes safety and optimal fueling. For further scientific guidance on endurance performance, consider resources like the Korey Stringer Institute at the University of Connecticut.

Frequently Asked Questions

No, for a 4-hour marathon, it is not recommended to drink only water. Over such a long duration, your body loses significant amounts of electrolytes, particularly sodium, through sweat. Replacing these with water alone can lead to a dangerous imbalance called hyponatremia.

The most effective method is to pre-determine your personal sweat rate during training. During the race, pay attention to thirst and aim for consistent fluid intake based on your calculated hourly needs. Your urine color (pale yellow) is also a good indicator, but less practical during the race itself.

The primary risk of overdrinking is hyponatremia, where blood sodium levels become dangerously low due to dilution. Symptoms include swelling, confusion, nausea, and headaches. The extra fluid can also cause a sloshing feeling in your stomach.

It's best to have a planned fluid intake schedule based on your personal needs rather than relying solely on aid stations. However, regular, small sips from aid station cups every few miles is a sound strategy, especially when supplementing with electrolytes.

Yes, for a 4-hour marathon, sports drinks are generally superior because they provide both carbohydrates for energy and electrolytes to replace salts lost in sweat. Water is sufficient for shorter, less intense runs, but sports drinks are recommended for longer endurance events.

Practice your hydration strategy during training. Consume small, frequent sips of fluid rather than chugging large amounts at once. This allows your stomach to absorb fluids more efficiently without feeling heavy or causing nausea.

A general guideline is to consume between 400 and 800 ml (or 14 to 27 ounces) of fluid per hour. However, this is highly individual and depends on your sweat rate, body size, and the weather. Your calculated sweat rate should be your personalized guide.