Marathon Fuel Calculator - Carb & Hydration Strategy
Avoid the dreaded 'bonk' at mile 20. This marathon nutrition calculator tells you exactly how many carbs and ounces of water you need per hour based on your body weight, pace, and finish time. Get a personalized fueling plan that prevents hitting the wall. Perfect for marathon runners, ultra runners, and anyone racing longer than 90 minutes. Free, science-backed, race-proven strategy. It helps you move from guesswork to a confident decision with clear inputs and readable output. Adjust values, compare scenarios, and share results quickly. Runs client-side in your browser, with no signup required. Built for speed, clarity, and repeat use. Method details for Marathon Fuel Calculator: The result model exposes each formula and equation, applies deterministic calculation steps, uses explicit decimal rounding, and keeps unit assumptions visible so outputs are auditable.
Pounds (lbs)
Kilograms (kg)
Your Fueling Strategy
Carbs Per Hour
60g
Water Per Hour
600ml
Total Duration
3:30
Hourly Breakdown
| Hour | Mile Marker | Carbs | Water |
|---|
💡 Fuel Options
Energy Gels: 20-25g carbs per gel (consume 2-3 per hour)
Sports Drinks: 15-20g carbs per 250ml (counts toward hydration)
Chews/Blocks: 8-10g carbs per piece (6-8 pieces per hour)
Bananas: ~25g carbs per medium banana
⚠️ Important Safety Information
These are general recommendations. Individual needs vary based on fitness level, weather conditions, heat acclimatization, and personal tolerance. Start fueling early (15-20 minutes into the race). Practice your fueling strategy during long training runs. Consult a sports nutritionist or physician for personalized advice.
How to Calculate Your Fueling Needs
- Enter your body weight - Heavier runners burn more calories
- Input target finish time - Determines how long you'll be running
- Review hourly needs - Grams of carbs and ounces of water per hour
- Plan your fuel sources - Energy gels (25g), chews (8-12g), sports drink
Why Fueling Strategy Matters
Your body can only store about 2000 calories as glycogen - enough for 90-120 minutes of running. After that, you must consume carbs or your pace will crater. This calculator uses the formula: Carbs/hour = 30-60g (easy) to 90g (trained) and Hydration = Body Weight × 0.4-0.8 oz/hour based on exercise science research.
The 'bonk' explained: When glycogen depletes, your body switches to fat metabolism, which is slower and less efficient. You'll feel dizzy, weak, and your pace will drop 2-3 minutes per mile. Proper fueling maintains blood glucose and delays glycogen depletion.
Pro tip: Practice your fueling strategy during long training runs. Your gut can be trained to absorb more carbs during exercise. Start with 30-40g/hour and gradually increase to 60-90g/hour over 8-12 weeks. Never try new fuel on race day.