How Much Water Should You Drink During a 20 Mile Run?

A 2% loss in body weight from dehydration can significantly impair athletic performance. Therefore, a well-defined hydration plan is crucial for a 20-mile run to prevent performance decline and health issues.

Quick Summary

Hydration needs for a 20-mile run vary based on individual factors like sweat rate and weather. Strategies involve pre-hydrating, electrolyte consumption, and post-run replenishment to support the body.

Key Points

• Personalize your intake: The exact amount of water needed varies based on individual sweat rate, body size, and environmental conditions.

• Combine water and electrolytes: For a 20-mile run, alternate between plain water and a sports drink to replace both fluids and essential minerals lost through sweat.

• Pre-hydrate effectively: Drink 16–20 ounces of fluid 2–3 hours before your run to start properly hydrated and give your body time to absorb it.

• Calculate your sweat rate: Use a test run to weigh yourself before and after, then adjust your hourly fluid intake based on your personal fluid loss.

• Replenish post-run: For every pound of body weight lost, consume 16–24 ounces of fluid after your run to support recovery.

• Carry your fluids: Use a hydration vest, belt, or handheld bottle to ensure consistent access to fluids, especially if running on a course without aid stations.

• Listen to your body's cues: While a hydration schedule is helpful, always drink when you feel thirsty. Pay attention to symptoms like fatigue or dark urine.

Why Your Hydration Strategy is Highly Individual

There is no single answer to the question of how much water to drink during a 20 mile run. The right amount varies based on personal and environmental factors. These factors include your sweat rate, body weight, the run's intensity and duration, and the weather conditions, especially temperature and humidity. Because of this, it is essential to create a personalized hydration strategy through testing during your training.

The Importance of Electrolytes

While water is important, drinking only water is not enough for a 20 mile run. Electrolytes, such as sodium, potassium, magnesium, and calcium, are lost when you sweat. These minerals are vital for fluid balance, muscle function, and nerve signaling. For runs over 60 minutes, electrolyte supplementation is needed to prevent hyponatremia, which is dangerously low blood sodium caused by drinking too much plain water.

Key Stages of Hydration for Your 20 Mile Run

Your hydration plan should involve three key stages: before, during, and after your run.

1. Pre-Run Hydration (The Day Before & Morning of):

• Day Before: Drink water consistently throughout the day before your long run. Aim for your normal daily fluid intake, about 2.7 to 3.7 liters for average adults.
• Morning Of: Consume 16–20 ounces of water or a sports drink 2–3 hours before starting. This gives the body time to absorb fluids and remove any excess.
• 15-20 Minutes Before: Sip another 6–8 ounces of fluid.

2. During-Run Hydration (Every 15–20 Minutes):

• Drink 5–10 fluid ounces (approximately 150–300 ml) of fluid every 15–20 minutes. This is a good starting guideline, but remember to adjust based on your personalized sweat rate.
• If you sweat heavily or run in hot weather, you may need to increase this to 6–8 ounces every 15–20 minutes.
• Alternate between plain water and a sports drink containing carbohydrates and electrolytes to keep a proper balance.

3. Post-Run Hydration (Crucial for Recovery):

• A simple way to measure post-run hydration is to weigh yourself before and after your run.
• For every pound of body weight lost, aim to replenish with 16–24 ounces of fluid.
• Continue drinking water and an electrolyte-rich beverage for several hours afterward to fully recover.

Calculating Your Personal Sweat Rate

To refine your hydration strategy, calculate your sweat rate during a test run.

1. Weigh yourself naked (A).
2. Run for one hour under similar conditions to your 20-mile run, keeping the intensity steady.
3. Measure the amount of fluid you drink during the run (Z).
4. Weigh yourself naked after the run (B).
5. Calculate your weight loss (C) by subtracting B from A.
6. Add the fluid consumed (Z) to your weight loss (C). This total is your sweat rate per hour.

Example calculation from Ironman.com:

• Pre-run weight: 150 lbs
• Post-run weight: 148 lbs
• Fluid intake: 24 oz
• Weight loss = 2 lbs (approx. 32 oz fluid)
• Sweat loss = 32 oz + 24 oz = 56 oz per hour.
• This runner would need to consume roughly 56 ounces of fluid per hour to match their sweat rate.

Hydration Comparisons: Water vs. Sports Drinks

For longer efforts like a 20-mile run, a combination approach is generally recommended. Here’s a comparison:

Feature	Plain Water	Sports Drink (with electrolytes)
Best for	Shorter runs (< 60 min), or to supplement electrolyte drinks on longer runs.	Runs longer than 60-90 minutes, especially in heat or high humidity.
Key benefit	Rehydrates without added sugars; essential for daily hydration.	Replenishes lost electrolytes (sodium, potassium) and provides carbohydrates for energy.
Risk of over-consumption	Hyponatremia (low blood sodium).	Potential for stomach distress if too concentrated; risk of over-sugaring.
Taste & motivation	Neutral; can become monotonous during long runs.	Varied flavors may encourage consistent drinking.
Availability	Widely available via bottles, taps, and fountains.	Available in pre-mixed bottles, powders, tablets, or gels.

Conclusion

Optimizing fluid intake during a 20-mile run is crucial for marathon training and safety. As a general rule, sip 5-10 ounces of fluid every 15-20 minutes, using a mix of water and electrolyte-rich sports drinks, especially for distances beyond an hour. A personalized strategy is most effective, determined by calculating your sweat rate and testing different fluid combinations during training. By being mindful of your body's signals and preparing a deliberate hydration plan, you can successfully complete your long runs feeling strong and well-fueled. Remember to drink consistently throughout the day leading up to your run and prioritize recovery with adequate fluid and electrolyte replenishment afterward.

Learn more about calculating your sweat rate with a simple home test.

Hydration Gear Options

To make your plan a reality, consider the following gear options for carrying fluids:

• Handheld water bottles: Great for runners who prefer simplicity and need easy access. Many come with straps for a secure grip.
• Hydration belts/waistpacks: Distributes weight around your hips and can hold multiple smaller bottles and nutrition.
• Hydration vests/packs: Ideal for carrying larger fluid volumes, with a reservoir and hose system for continuous access. Offers additional storage space for fuel and gear.

These tools help ensure that consistent hydration is always within reach, preventing the temptation to skip a drink and risk performance decline.

Practical Hydration Tips for Your Long Run

• Set a timer: A watch alarm can remind you to take a sip every 15-20 minutes, preventing you from zoning out and forgetting to drink.
• Plan your route: On training runs, build your route around accessible water fountains or loop back to your car for refills.
• Consider the conditions: In hotter or more humid weather, you may need to increase your fluid intake to account for higher sweat loss. In colder weather, remember that you still lose fluids, even if you don't feel as thirsty.

By systematically approaching your hydration, you can ensure your body is always in the optimal state for performance and recovery, especially when pushing through the final miles of a long 20-mile run.

Frequently Asked Questions

How often should I drink during a 20-mile run?

You should aim to drink 5–10 fluid ounces (about 1–2 big sips) every 15–20 minutes to maintain steady hydration and avoid over-consuming fluid at once.

Is water enough for a 20-mile run, or do I need electrolytes?

For runs lasting over an hour, including a 20-miler, you should consume fluids with electrolytes. Electrolytes are essential minerals lost in sweat and help prevent conditions like hyponatremia.

What happens if I drink too much water during a long run?

Over-consuming plain water can lead to hyponatremia, a dangerous condition where blood sodium levels become too low. Symptoms can include nausea, bloating, and confusion.

How can I tell if I am properly hydrated before my run?

A good indicator is the color of your urine. It should be a pale yellow color, similar to lemonade. If it is dark yellow, you need to increase your fluid intake.

What is a 'salty sweater' and how does it affect hydration?

A 'salty sweater' is a runner who loses a high concentration of sodium in their sweat. These individuals need to be extra mindful of replacing electrolytes during a long run to avoid imbalances.

How does weather affect my hydration needs for a 20-mile run?

In hot and humid conditions, your sweat rate will be higher, requiring you to increase your fluid intake. In colder weather, you still lose fluids, so maintaining consistent intake is important even if you don't feel as thirsty.

Should I drink before, during, and after a 20-mile run?

Yes, a proper hydration plan includes all three phases: pre-loading fluid the day and morning before, consistently drinking during the run, and actively replenishing fluids and electrolytes during recovery.