42calMarathon Fueling CalculatorMarathon 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
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 an increasingly important fuel source. Glycogen stores are limited, so taking in carbohydrates during prolonged exercise can help maintain performance. Current sports-nutrition guidance commonly uses 30-60 grams per hour for events lasting 1-2.5 hours and up to 90 grams per hour for longer events when multiple transportable carbohydrates are used. The higher end should be practiced in training rather than attempted for the first time on race day. The planner adjusts its target based on duration, intensity, and gut tolerance.
Sources and review
Last reviewed: September 18, 2026 · Use this plan as a starting point and test it in training
- Carbohydrate Intake During Exercise — Asker Jeukendrup, Sports Medicine (2014)
- Nutrition and Athletic Performance — Academy of Nutrition and Dietetics, Dietitians of Canada, and ACSM (2016)
Frequently Asked Questions
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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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