Marathon Fueling Calculator
Create your personalized race-day fueling plan with science-backed carb, fluid, sodium, and caffeine targets
Proper fueling typically improves time-trial performance by ~2-6% vs. ad-lib fueling in studies
Use this free marathon fueling calculator to create a personalized race nutrition plan. For marathons, most runners thrive on ~60–90 g/h carbs. Trained guts using glucose+fructose blends may tolerate more—practice this in long runs. Aim for ~0.4–0.8 L/h fluids based on your measured sweat rate and conditions. Start fueling in the first 20–30 minutes and avoid over-drinking to prevent hyponatremia.
Updated: January 2025 | Author: Pace Perfect Sports Science Team | Review: Sports Nutritionist reviewed | Methodology: Based on ACSM guidelines and endurance nutrition research | For educational purposes; consult healthcare providers for medical advice.
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Why Race-Day Fueling Matters
Proper fueling prevents "hitting the wall" by maintaining blood glucose and delaying glycogen depletion. Research shows properly fueled runners improve performance by ~2-6% compared to ad-lib approaches in time-trial studies.
Key Fueling Principles:
- Carbs: 60-90 g/h for most marathoners; trained guts may handle more with glucose+fructose blends
- Fluids: 0.4-0.8 L/h guided by sweat rate and conditions
- Sodium: ~300-600 mg/h (adjust to sweat sodium if known)
- Timing: Start fueling in first 20-30 minutes
Performance Benefits:
- Maintains blood glucose throughout race
- Delays glycogen depletion and fatigue
- Prevents hyponatremia (low blood sodium)
- Supports consistent pacing
- Practice in training: Test your fueling strategy during long runs, not for the first time on race day
- Avoid over-drinking: Hyponatremia symptoms (nausea, confusion, swelling) → seek medical help immediately
- Carry backup fuel: Always have extra gels/chews in case aid stations run out
- Adjust for conditions: Hot/humid weather increases fluid and sodium needs
📊 Methods & Evidence Panel (click to expand)
Calculations Used:
- Carbs per hour: Duration-based targets of 60-90 g/h for marathons (not body-weight-based)
- Fluids per hour: Sweat-rate-guided (typically 0.4-0.8 L/h); Formula: Fluid (L/h) = (pre-weight - post-weight + fluid consumed) / hours
- Sodium per hour: mg/h = (mg/L concentration × L/h fluid intake)
- Caffeine total: 3-6 mg/kg body weight (e.g., ~60 min pre-race + optional mid-race top-ups)
- Timing intervals: Typically every 20-30 min (or 3-5 km) for carbs/fluids
Evidence Ranges:
- Carbohydrate intake: 60-90 g/h for marathons; some trained athletes with glucose+fructose blends can manage 90-120 g/h
- Fluid intake: 0.4-0.8 L/h based on individual sweat rates (typically 0.5-2.0 L/h depending on conditions)
- Sodium concentration: 230-690 mg/L in bottles (10-30 mmol/L); range accounts for most runners' sweat sodium (10-90 mmol/L, some higher)
- Caffeine dosing: 3-6 mg/kg total for ergogenic benefit; timing ~60 min pre-race optimal
- Start timing: Begin fueling within first 20-30 min (count pre-start top-off 10-15 min before gun as "early race fuel")
Key References:
- Jeukendrup, A. (2014) - A step towards personalized sports nutrition: carbohydrate intake during exercise, Sports Med
- ACSM Position Stand (2016) - Nutrition and Athletic Performance
- Burke et al. (2019) - Carbohydrates for training and competition, J Sports Sci
- Sawka et al. (2007) - Exercise and fluid replacement, ACSM Position Stand
- Stellingwerff & Cox (2014) - Systematic review on carbohydrate intake in endurance sports
Last Updated: January 2025 | Reviewed by: Sports Nutrition Science Team
🍬 Common Fueling Products (Reference Guide)
Use these examples to build your race-day fueling plan. Amounts shown are approximate:
Carbohydrate Sources:
- 1 energy gel ≈ 20-25g carbs
- 1 serving chews (6 pieces) ≈ 24g carbs
- 500mL sports drink ≈ 30-35g carbs
- 1 banana ≈ 25-30g carbs
- 1 stroopwafel ≈ 20-22g carbs
Sodium & Electrolyte Sources:
- Electrolyte tablet (500mL) ≈ 300-500mg sodium
- Sports drink (500mL) ≈ 200-400mg sodium
- Salt stick capsule ≈ 215mg sodium
- Salted gels ≈ 50-100mg sodium
Caffeine Sources:
- Caffeinated gel ≈ 20-40mg caffeine
- Coffee (8oz) ≈ 95mg caffeine
- Energy chews (6 pieces) ≈ 50mg caffeine
💡 Pro tip: Glucose+fructose blends (2:1 ratio) allow higher carb absorption rates for trained guts. Practice with your chosen products during long training runs.
🌅 Pre-Race Fueling (2-4 Hours Before)
- 2-3 hours before: 500-600mL fluid with easily digestible carbs (toast, banana, oatmeal)
- 1 hour before: Small carb top-off (gel, banana) + sips of fluid
- 10-15 min before gun: Optional final gel or chews (counts as "early race fuel")
- Caffeine timing: If using, consume ~60 min before start for peak effect
Complete Your Race Preparation
Maximize your marathon performance with our full suite of science-backed tools:
🍞 Carb Loading Calculator
Plan your 48-72h pre-race carbohydrate loading strategy
⏱️ Marathon Pacing Calculator
Determine your optimal race pace and split times
🎯 Ready for Your Complete Marathon Strategy?
Race-day fueling is just one piece of the puzzle. Get a personalized training plan that includes:
Hour-by-hour carb strategy
Integrated with your taper
Mile-by-mile adjustments
Personalized to your fitness
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