Half Marathon & Marathon Fueling Calculator

Why fueling matters for half marathon and marathon

Your body stores roughly 400–500 g of glycogen — about 1,600–2,000 kcal — split between liver and muscle. Running burns roughly 60–80 kcal per mile (depending on body weight and pace), most of it from carbohydrate at race intensity. The math is unforgiving: a marathon at race effort can demand 2,500+ kcal of substrate, and once muscle glycogen runs out, pace falls off a cliff. The folk term is "hitting the wall." The physiological term is glycogen depletion combined with falling blood glucose, and the entire purpose of in-race fueling is to prevent it.

Half marathons sit at the edge of this. Most runners can finish 13.1 miles on stored glycogen and a single pre-race breakfast, but for anyone running longer than ~1:30, fueling during the race is the difference between a steady second half and a painful one. For marathoners, fueling is non-negotiable.

For background reading, see our guide to endurance fueling basics and our overview of carbohydrates for runners.

Carbohydrates during the race

The current ACSM/AND/Dietitians of Canada joint position statement (Thomas, Erdman, & Burke, 2016) lays out the tiered targets we use in this calculator:

• Under 1 hour: water suffices. A carbohydrate mouth rinse may give a small performance boost.
• 1–2.5 hours: 30–60 g of carbs per hour. Glucose-only products (most simple gels) work fine in this range.
• Over 2.5 hours: 60–90 g per hour, requiring multiple transportable carbohydrates — a glucose + fructose blend in roughly 2:1 ratio. The fructose uses a separate gut transporter (GLUT5) that bypasses the saturated glucose pathway.

Recent work in trained marathoners has pushed this further. Viribay et al. (2020) and Urdampilleta et al. (2020) showed that intakes of 90–120 g/hr are well tolerated and reduce muscle damage and post-race fatigue — but only with prior gut training. Untrained guts will produce GI distress at those rates. This is why the calculator asks about your gut training history: it caps the recommendation at what your gut has actually practiced.

Training your gut

The intestinal absorption of carbohydrates is genuinely adaptable. Cox et al. (2010) and Jeukendrup (2017) demonstrated that 4–6 weeks of high-carb intake during long runs increases both transporter expression and tolerance. The practical rule: train your fueling exactly as you plan to race it. If your race target is 80 g/hr, your 18+ mile long runs in the final training block should hit that rate. Race day is not the time to discover that 80 g/hr makes you nauseous.

Carb-loading: what's changed

The week-long depletion-then-load protocol of the 1970s is no longer recommended. Current guidance is much simpler:

• Marathon: 8–12 g/kg/day for 24–48 hours pre-race. The calculator anchors at 10 g/kg/day.
• Half marathon: 7–8 g/kg/day for the day before. Less critical than for the marathon.
• Reduce fiber and fat in the last 24 hours to limit GI bulk on race morning.
• Hydrate well during loading — each gram of glycogen pulls roughly 3 g of water with it. Some runners gain 1–2 kg from loading; that's expected.

For practical food choices, see our guide to what to eat before a run.

The pre-race meal

The Academy of Nutrition and Dietetics recommends 1–4 g of carbohydrate per kg body weight, eaten 1–4 hours pre-race. The calculator gives a 2–4 g/kg target for the standard 3-hour window. Pair with fluid (5–10 ml/kg). Pick familiar foods — this is not the morning to experiment.

See our guide on how long to wait between eating and running for individual timing tradeoffs.

Fluid: enough but not too much

Sweat losses in running typically range from 400 to 1,500 ml/hr depending on body size, intensity, and climate. The calculator's 400–900 ml/hr range is what most runners can actually drink without GI distress, scaled by body weight and climate. The goal is to limit dehydration to ~2% of body weight, not to fully replace losses in real time — that's usually impossible at race intensity.

Don't overdrink. Exercise-associated hyponatremia from excessive plain-water intake is a documented cause of marathon collapses. The 2015 Wilderness Medical Society statement identifies it as a serious risk for slower runners who drink at every aid station. Drinking to thirst is a reasonable default; sodium-containing drinks are safer than plain water for any race over 90 minutes.

For sports-drink choice, see our best sports drinks for runners review.

Sodium

For races over 1 hour, 300–700 mg of sodium per hour is the published range. Heavy/salty sweaters (visible salt residue on clothing or skin) and hot weather push toward the top.

Sources: most sports drinks deliver 200–500 mg per 16 oz, most endurance gels add 50–200 mg, and salt capsules typically contain 200–400 mg each. Mix and match to hit your target.

Recovery: the first 60 minutes

Glycogen resynthesis is fastest in the first hour after exercise. The targets:

• Carbohydrates: 1–1.2 g/kg in the first hour (Burke et al., 2017).
• Protein:0.3 g/kg post-run (Phillips & Van Loon, 2011) supports muscle protein synthesis. 20–40 g total for most runners.
• Fluid: 1.25–1.5 L per kg of body weight lost during the race, sipped over 4–6 hours.
• Caffeine: some evidence for accelerated glycogen replacement when paired with carbs (3 mg/kg). Optional.

See our post-race recovery tips for the broader picture, and our piece on the effects of caffeine on running performance if you're considering caffeinated gels.

What this calculator is — and isn't

This calculator gives you population-level targets calibrated to your inputs. It is built on peer-reviewed guidelines from the American College of Sports Medicine, the Academy of Nutrition and Dietetics, the International Society of Sports Nutrition, and the published research of Asker Jeukendrup, Louise Burke, and others. It is educational, not medical advice. If you have diabetes, a history of GI distress in races, food allergies, or any medical condition that affects fueling, work with a registered sports dietitian who can build a fully individualized plan.

One more honest caveat: every runner's gut is different. Even with perfect targets, race-day fueling only works if you've practiced it. Treat the targets as the goal of training, not the start of training.

Related running articles

• The Basics of Endurance Fueling — overall framework for before, during, and after.
• Carbohydrates for Runners — daily carb intake and training adaptation.
• What to Eat Before a Run — specific food choices and timing.
• How Long Should You Wait to Run After Eating?
• Best Sports Drinks for Runners
• Effects of Caffeine on Running Performance
• Meal Planning for Runners
• Post-Race Recovery Tips for Runners

Related tools

• Race time predictor — calibrate your goal time for this calculator.
• VDOT calculator — training paces for the long runs where you practice fueling.
• Running training load calculator — plan the weekly mileage that prepares you for the race you're fueling.
• Pace calculator — split the race into pacing chunks to anchor fueling cadence.
• Heart rate zone calculator — intensity control on long runs (carb burn rises sharply above zone 2).

References

1. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501–528.
2. Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(S1), 25–33.
3. Jeukendrup, A. (2017). Training the gut for athletes. Sports Medicine, 47(S1), 101–110.
4. Kerksick, C. M., Arent, S., Schoenfeld, B. J., et al. (2017). International Society of Sports Nutrition position stand: nutrient timing. Journal of the International Society of Sports Nutrition, 14(1), 33.
5. Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(S1), S17–S27.
6. Burke, L. M., van Loon, L. J. C., & Hawley, J. A. (2017). Postexercise muscle glycogen resynthesis in humans. Journal of Applied Physiology, 122(5), 1055–1067.
7. Viribay, A., Arribalzaga, S., Mielgo-Ayuso, J., et al. (2020). Effects of 120 g/h of carbohydrates intake during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients, 12(5), 1367.
8. Urdampilleta, A., Arribalzaga, S., Viribay, A., et al. (2020). Effects of 120 vs. 60 and 90 g/h carbohydrate intake during a trail marathon on neuromuscular function and high intensity run capacity recovery. Nutrients, 12(7), 2094.
9. Cox, G. R., Clark, S. A., Cox, A. J., et al. (2010). Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. Journal of Applied Physiology, 109(1), 126–134.
10. Phillips, S. M., & Van Loon, L. J. C. (2011). Dietary protein for athletes: from requirements to optimum adaptation. Journal of Sports Sciences, 29(S1), S29–S38.
11. Hew-Butler, T., Rosner, M. H., Fowkes-Godek, S., et al. (2015). Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine, 25(4), 303–320.