42calMarathon Fueling CalculatorRace Nutrition
Marathon Fueling Calculator
Plan what to carry, when to fuel, and how much to take.
01Your run
Trained guts can handle up to 90g/hr carbs vs 60g/hr for beginners.
02Your fuel
Use the carbohydrate amount on your label. One serving can be a gel, a chew portion, or a bottle.
Your race-day card
Carbs / hour
60g/h
Total
210g
Pack
9
- 0:1225g
- 0:3525g
- 0:5825g
- 1:2225g
- 1:4525g
- 2:0825g
- 2:3225g
- 2:5525g
- 3:18≈ 0.4 serving10g
Optional before the start: 1 extra serving (25g). Not included in race totals.
Use 10g carbs from the last serving; carry the whole pack.
Fluids: 500–700 ml/h · Sodium: 500–700 mg/h
Drink to thirst. Adjust fluids to your sweat rate and conditions; do not force the estimated total. Count sports-drink carbs within this plan, not on top of it.
Rehearse this plan on a long run. For targets above 60g/h, use glucose–fructose products you have practiced with.
Carbohydrate intake recommendations are based on current sports nutrition research, with rates adjusted for effort duration and intensity.
Gut training is crucial—the GI system can be trained to absorb more carbs with regular practice, increasing from ~60g/hr to up to 90g/hr.
Carb Rate Guidelines:
- • <60 min: Water sufficient, minimal fueling
- • 60-90 min: 30-45g/hr
- • 90-150 min: 45-60g/hr
- • >150 min: 60-90g/hr
Share this tool with your training group or embed it on your coaching website.
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How to use this fuel planner
- Race Day mode: Enter your expected race duration, conditions, and fuel preferences to get a personalized fueling timeline.
- Training Week mode: Enter days until your race and body weight to get carb loading targets.
- Practice your plan: Always test your nutrition strategy during long training runs before race day.
- Adjust based on feel: These are starting points—everyone responds differently to nutrition during exercise.
How many carbs per hour
| Race length | Carbs per hour |
|---|---|
| Under 1 hour | None needed |
| 1 to 1.5 hours | 30–45 g |
| 1.5 to 2.5 hours | 45–60 g |
| Over 2.5 hours | 60–90 g |
A 4:00:00 marathon at 75 g per hour is 300 g of carbs, or 12 gels of 25 g.
The Science Behind Race Nutrition
During endurance exercise, your body burns a mix of fat and carbohydrates. At marathon pace and faster, carbohydrates become the dominant fuel source, but your body can only store about 2,000 calories of glycogen — enough for roughly 90-120 minutes of hard running. After that, you need exogenous carbohydrates to maintain pace. Current sports science recommends 30-60 grams of carbs per hour for events lasting 1-2.5 hours, and up to 60-90 grams per hour for longer events when using a glucose-fructose combination. The planner adjusts these targets based on your intensity, duration, and gut tolerance to find a sustainable intake rate.
Frequently Asked Questions
Related Tools
- Pace Calculator — Estimate your finish time to plan fuel timing
- Difficulty Analyzer — Factor in course difficulty when planning nutrition
- Training Pace Zones — Practice fueling at your race-specific training paces
- Race Time Predictor — Predict your finish time to estimate race duration
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