Carb Fuel Calculator
Plan your carbohydrate fueling strategy for race day.
Quick Presets
Race Duration
Enables personalized g/kg targets for pre-race meals and carb loading.
Nail Your Race-Day Fueling Strategy
Bonking during a marathon is almost always a fueling failure, not a fitness failure. Your body stores roughly 2,000 kcal of glycogen — enough for about 90 minutes of hard running. After that, you need exogenous carbohydrates to maintain pace. The Carb Fuel Calculator tells you exactly how many grams per hour to consume, how many gels to carry, and when to take them — based on current sports nutrition research and the duration-based thresholds used by elite fueling protocols.
What Is a Carb Fuel Calculator?
This calculator applies the evidence-based carbohydrate intake guidelines from sports nutrition research (MSD §9). For durations under 2.5 hours, the recommended range is 30–60 g/hr. For durations of 2.5 hours or more, the range increases to 60–90 g/hr to match the higher glycogen depletion rate. The calculator converts these hourly targets into total carb needs and a practical gel schedule based on standard 25g energy gels.
Why Use a Carb Fuel Calculator?
- Prevent bonking by planning exact carb intake before race day
- Know how many gels to carry and when to take each one
- Understand the 2.5-hour threshold where fueling requirements increase
- Avoid GI distress by not over-fueling during shorter events
- Plan gel timing to spread intake evenly across the race
- Free, instant, based on current sports nutrition guidelines
How to Use the Carb Fuel Calculator
- 1
Choose a preset or enter your duration
Select a race scenario (5K, 10K, Half Marathon, Marathon, Ultra) or enter your expected duration in hours and minutes.
- 2
Click "Calculate Fuel Plan"
The calculator instantly determines whether you fall into the short-duration (30–60 g/hr) or long-duration (60–90 g/hr) protocol based on the 2.5-hour threshold.
- 3
Review your fuel plan
See hourly carb targets, total carb needs, number of gels required, and recommended gel intervals. The chart visualizes hourly vs. total intake at a glance.
- 4
Practice in training
Use the gel schedule during long training runs to train your gut. Start with the minimum recommendation and work up to the maximum over several weeks.
Who Uses a Carb Fuel Calculator?
Marathon runners
A 4-hour marathoner needs 240–360g of carbs during the race. Without a plan, most runners under-fuel by 30–50%, leading to the dreaded "wall" at mile 20.
Half marathon runners
A sub-2-hour half marathon falls in the 30–60 g/hr range. Many runners skip fueling entirely for halfs, but 1–2 gels can maintain pace in the final miles.
Ultra & trail runners
Events over 4 hours require sustained 60–90 g/hr intake with mixed carb sources. The calculator helps plan total carb needs for aid-station strategies.
Coaches & nutritionists
Provide athletes with personalised fueling plans based on projected finish times — easy to update as race fitness improves.
First-time marathoners
Answers the #1 beginner question: "How many gels do I need?" Shows exactly how many and when to take them.
Triathletes
Plan carb intake across bike and run legs separately, accounting for the different fueling windows each discipline allows.
Under the Hood
The evidence-based fueling thresholds and gel math behind every recommendation.
Duration Threshold (2.5 Hours)
Below 2.5 hours, muscle glycogen can supplement incoming carbohydrates, so 30–60 g/hr is sufficient. At 2.5+ hours, glycogen depletion becomes significant and the body needs 60–90 g/hr from external sources to maintain blood glucose and performance.
Hourly Range Calculation
The engine applies a simple threshold: if durationHours < 2.5 then hourlyLow = 30, hourlyHigh = 60; else hourlyLow = 60, hourlyHigh = 90. Total carbs = hourly rate × duration.
Gel Math
Standard energy gels contain 25g of carbohydrate. The calculator divides total carb needs by 25 to compute gel count, then divides duration by gel count to compute recommended timing intervals.
Dual-Transport Carbohydrates
Research shows that consuming glucose and fructose together (2:1 ratio) enables absorption rates above 60 g/hr — up to ~90–100 g/hr. Products like Maurten and SIS Beta Fuel use this dual-transport mechanism. For events over 2.5 hours, dual-transport formulas are recommended.
Privacy
All calculations run entirely in your browser. Your duration and fueling data are never transmitted to any server.
Example Scenarios
Real inputs and the exact output from the engine — so you know what to carry on race day.
1:45 Half Marathon
A 1 hour 45 minute half marathon falls below the 2.5-hour threshold. The engine prescribes 30–60 g/hr, giving a total of 53–105g of carbohydrates. That's 2–4 standard gels — one every 25–50 minutes.
1.75 hr → 30–60 g/hr · 53–105g total
4:00 Marathon
A 4-hour marathon crosses the 2.5-hour threshold. The engine prescribes 60–90 g/hr, giving a total of 240–360g. That's 10–14 gels — one every 17–24 minutes. At this volume, dual-transport carbohydrate products are recommended to hit the upper range without GI distress.
4.0 hr → 60–90 g/hr · 240–360g total
6:00 Ultra
A 6-hour event at the 60–90 g/hr rate requires 360–540g of total carbohydrates. That's 14–22 gels, or more practically, a mix of gels, chews, sports drinks, and solid food planned across aid stations.
6.0 hr → 60–90 g/hr · 360–540g total
Research & References
Fueling recommendations in this calculator follow consensus guidelines from sports nutrition research.
- Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise, 48(3), 543–568.
- Jeukendrup, A. E. (2014). A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise. Sports Medicine, 44(S1), 25–33.
- Jeukendrup, A. E. (2011). Nutrition for Endurance Sports: Marathon, Triathlon, and Road Cycling. Journal of Sports Sciences, 29(S1), S91–S99.
Frequently Asked Questions
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