How Much Fluid Should a Runner Drink During Running?

longlongendurance
Aug 3, 2025
3 min read

Why Hydration Matters

Hydration impacts everything from your cardiovascular system and body temperature regulation to muscle function and recovery. Poor hydration can lead to fatigue, cramps, decreased performance, and even dangerous conditions like heatstroke or hyponatremia.

General Fluid Intake Guidelines

While hydration needs vary based on weather, intensity, duration, and individual sweat rates, here’s a helpful baseline:

Before Running: Aim for 16-20 ounces (470-590 ml) of water about 2 hours before starting.
During Running: Generally, consume about 5-10 ounces (150-300 ml) every 15-20 minutes.
After Running: Drink at least 16-24 ounces (470-710 ml) for every pound of body weight lost during your run.

Personalizing Your Fluid Intake

One-size-fits-all doesn't apply here. Consider these key factors:

Sweat Rate: Measure your sweat rate by weighing yourself before and after a typical run. This helps estimate your personalized fluid loss and determine how much you should drink to stay hydrated.
Weather Conditions: Heat and humidity significantly increase fluid needs. Adjust intake upward in warmer, more humid conditions.
Intensity and Duration: Longer and more intense runs require increased fluid and electrolyte intake.

How to Determine Your Sweat Rate

The most accurate method to determine your sweat rate and electrolyte (including sodium) concentration is through laboratory testing. Lab tests typically involve Sweat Composition Analysis and Precise Sweat Rate Measurement. But you can estimate your sweat rate following:

Note the temperature and humidity of the environment you're running in.
Weigh yourself without clothes immediately before running.
Run at an easy pace for one hour, preferably without drinking fluids. If you do drink, note exactly how much fluid you consumed.
If you use the bathroom, estimate the volume of urine output.
Immediately after running, weigh yourself again without clothes.
Calculate fluid loss: every pound lost equals approximately 15.5 ounces of fluid.
Adjust for any fluids consumed or urine output during the run.

Example:

Pre-run weight: 150 lbs
Post-run weight: 149 lbs (1 lb or approximately 15.5 ounces lost)
Fluid consumed during run: 8 ounces
Total fluid lost: 15.5 ounces + 8 ounces = 23.5 ounces
Run duration: 1 hour
Sweat rate: 23.5 ounces/hour

Based on your sweat rate and the temperature/humidity conditions of future runs, you can better estimate your hydration needs.

Electrolytes: More Than Just Water

For runs lasting more than an hour or conducted in hot conditions, water alone isn't enough. Electrolytes like sodium, potassium, and magnesium are crucial. Calculate sodium intake based on sweat rate; typically, you might need around 300-700 mg of sodium per hour during prolonged or intense exercise. If you lose around 2 pounds of sweat per hour (common during intense or hot conditions), you might lose around 800–1400 mg of sodium. This variability highlights why it's important to personalize your sodium intake based on your individual sweat rate and sodium losses, especially during long runs or races.

Signs You're Not Drinking Enough

Pay attention to your body's signals:

Thirst (though it's already a sign of mild dehydration)
Dark urine
Dizziness or headache
Excessive fatigue or muscle cramps

Avoiding Overhydration

Believe it or not, overhydration can be dangerous. Drinking excessively without adequate electrolytes can lead to hyponatremia, a potentially life-threatening condition. Signs include nausea, swelling, and confusion. Stick to measured fluid intake and include electrolytes.

Practical Hydration Tips for Runners

Carry a hydration pack or handheld water bottle during long runs.
Plan routes with water stops.
Use sports drinks or electrolyte tablets during extended efforts.
Practice your hydration strategy in training, not just on race day.

Conclusion

Proper hydration is fundamental to safe, enjoyable, and successful running. Listen to your body, adjust according to conditions, and find a routine that keeps you consistently performing your best.