Key Takeaways

  • The Galpin Equation recommends drinking your body weight (kg) × 2 mL every 15 minutes during exercise — or body weight (lbs) ÷ 30 in fluid ounces.[1]
  • Even a 2% reduction in body water can measurably reduce performance, reaction time, and focus during training.[2]
  • For sessions over 60 minutes, adding electrolytes (sodium, potassium) to your fluid supports sustained output and fluid absorption.[3]

How the Galpin Equation Works

Metric: mL per 15 min = Body Weight (kg) × 2

Imperial: oz per 15 min = Body Weight (lbs) ÷ 30

Developed by Dr. Andy Galpin based on exercise physiology research

The equation provides a simple, body-weight-scaled guideline for fluid consumption during demanding physical activity. It is not intended for general daily hydration — for that, see the Daily Water Intake Calculator.[1]

The formula is based on the principle that larger individuals produce more metabolic heat and sweat more during exercise, requiring proportionally more fluid to maintain performance.[2]

This calculator extends the base equation with adjustments for exercise intensity and environmental conditions, which significantly affect sweat rate and fluid needs.

Intensity & Environment Adjustments

The base Galpin Equation assumes moderate-intensity exercise in a temperate environment. Real-world training varies widely, so this calculator applies evidence-based multipliers:

  • Low intensity (0.8×) — stretching, light yoga, warm-up drills
  • Moderate intensity (1.0×) — steady-state running, weightlifting
  • High intensity (1.2×) — HIIT, circuit training, sprints
  • Extreme (1.5×) — competition, prolonged high-output sessions

Hot and humid environments can increase sweat losses by 50% or more, which is why the environmental multiplier can reach 1.5× for hot-humid conditions.[3]

Track your workout energy expenditure using our AMR Calculator to correlate fluid needs with calorie output.

Limitations and Important Notes

  • Individual sweat rates vary significantly — the Galpin Equation provides a starting guideline, not an exact prescription.
  • Factors like altitude, clothing, acclimatization status, and individual physiology are not captured by this formula.
  • Over-hydration (hyponatremia) is also a concern during very long events — balance fluid intake with electrolyte replenishment.
  • This tool is for informational and journaling purposes only. It does not constitute personalized advice.

Frequently Asked Questions

Sources & References

  1. Hydration During Exercise — The Galpin Equation. Andy Galpin, PhD. Ask Andy Galpin (2023)
  2. Optimize Your Water Quality and Intake for Health. Andrew Huberman, PhD. Huberman Lab Newsletter (2023)
  3. Hydration and Physical Performance. Various. PMC (2025)
Manish Kumar
Manish Kumar

Certified Personal Trainer & Sports Nutritionist

NASM-CPTCertified Sports Nutritionist10+ Years Experience500+ Clients Coached

NASM-certified fitness and nutrition coach with over 10 years of hands-on experience helping people build strength, lose fat, and live healthier lives. Specializing in gym-based workouts with a strong focus on lifting technique, biomechanics, and practical exercise science. Through FitLifeRegime, sharing the tools, tips, and insights that have worked for hundreds of clients — helping you start your own fitness journey with confidence and clarity.

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Certified Personal Trainer & Sports Nutritionist from the National Academy of Sports Medicine

This tool is for informational and journaling purposes only.

I am NOT a doctor.