How many carbs per hour do I need during a half marathon or marathon?

It depends on your goal. To finish comfortably, 20–40 g/hr is plenty for a half marathon and 30–50 g/hr for a marathon. To race a strong time, most amateur runners successfully use 30–60 g/hr (half) or 45–70 g/hr (marathon). To maximize performance with a trained gut, 60–90 g/hr is supported by research. The aggressive 90–120 g/hr you may see online is the elite ceiling and requires weeks of gut training — it is not the right default for most runners.

I'm a first-time half-marathoner. How much should I fuel?

Pick 'Finish comfortably' and 'Beginner' in the calculator. For a typical first half marathon (1:45–2:15 finish), 30–40 g/hr — about two gels for the day — is enough to finish strong. Take the first gel around 30 minutes in and the second around 70 minutes in. Practice this exact pattern on at least three long runs before race day.

What are 'multiple transportable carbohydrates'?

When carb intake exceeds about 60 g/hr, the gut's glucose transporter (SGLT1) becomes saturated. To absorb more, you need a blend of glucose and fructose — typically a 2:1 ratio — which uses a separate transporter (GLUT5). Most modern endurance gels and sports drinks already use this blend. If you're targeting more than 60 g/hr, check the label.

How much should I drink during a marathon?

Most runners do well with 400–800 ml (14–27 oz) per hour, sipped steadily across aid stations rather than gulped. Body size and climate shift the target — a larger runner in heat needs the upper end, a smaller runner in cool weather the lower. Drink to thirst as a reasonable default; if you're losing more than 2% of body weight by the finish, drink more next time. Don't overdrink — hyponatremia from excessive plain-water intake is a documented marathon hazard.

How do I carb-load before a race?

For events longer than 90 minutes, the current ACSM/AND/DC position is 8–12 g of carbohydrate per kg of body weight per day for 24–72 hours pre-race. For most marathoners, 24–48 hours at ~10 g/kg/day is sufficient (the older 7-day depletion-then-load protocol is no longer recommended). Reduce fiber and fat in the last 24 hours to limit GI bulk. Hydrate well — every gram of stored glycogen pulls water with it.

What should I eat the morning of the race?

Aim for 1–4 g of carbohydrate per kg body weight, eaten 3–4 hours before the start. For a 70 kg runner that's roughly 70–280 g of carbs — typical race-morning meals land around 100–150 g. Choose familiar, low-fat, low-fiber foods (bagel + jam, oatmeal, white rice, banana). Pair with 5–10 ml of fluid per kg body weight. A small carb top-up (25–50 g) 15–30 minutes before the start is fine if you tolerate it.

Why does the calculator ask about my experience level?

The gut adapts to fueling rates over weeks of practice. A runner who has never taken more than 30 g/hr in training will get GI distress trying 90 g/hr on race day. Beginner caps the recommendation at 60 g/hr (where most amateurs start), Intermediate at 75 g/hr (a comfortable rate for runners who've practiced gels in long runs), and Advanced lifts the cap to ~110 g/hr (only relevant for runners who've deliberately trained their gut at high rates).

How much sodium do I need during a race?

For events over 1 hour, 300–700 mg of sodium per hour is the published range, with heavy or 'salty' sweaters and hot conditions pushing toward the top. Most sports drinks contain 200–500 mg per 16 oz, so a runner who drinks the recommended 600 ml/hr of sports drink already gets a meaningful portion. Salt capsules or fortified gels can fill any gap. If you regularly cramp late in races, try the higher end.

What about fueling for shorter races (10K, 5K)?

For races under 60 minutes, water is sufficient and carbs during the race offer minimal benefit (a mouth rinse with carbohydrate may give a small performance boost). Pre-race fueling matters more than mid-race fueling for these distances. This calculator is built for half marathon and marathon — for shorter races, a simple pre-race breakfast and water is enough.

Half Marathon & Marathon Fueling Calculator: Carbs, Fluid, Sodium

When	Target	Notes
2 days before	~726 g/day	Carb load. Reduce fiber and fat. Hydrate well.
3–4 hr before	145–290 g carbs	Plus ~500 ml fluid. Familiar foods, low fat, low fiber.
15–30 min before	25–50 g carbs	Optional gel or sports drink to top off.

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.

How to read the three plans

One of the most common mistakes in race-fueling articles online is presenting a single number — usually the elite ceiling — as though every runner should aim for it. The reality is that the right plan depends on your goal for the day, your experience, and what your gut has practiced. The calculator returns three plans side-by-side so you can pick the one that fits.

Conservative — fewer gels, lower stomach risk. A great choice for first-timers, runners with prior GI issues, or anyone whose primary goal is to enjoy the race and finish.
Recommended— the middle of your goal range and the option most amateur runners successfully use. This is the "most common approach" for your inputs.
Aggressive— the top of your goal range. Useful for runners chasing a PR who have practiced this exact fueling rate in training. Skip it if you haven't.

Carbohydrates during the race

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

Under 1 hour: water suffices. Carbs during the race add little. A carbohydrate mouth rinse may give a small performance boost.
1–2.5 hours: 30–60 g of carbs per hour for most runners. Glucose-only products (typical simple gels) work fine in this range. Many half-marathoners successfully race on the lower end — 30 g/hr is roughly one gel per hour.
Over 2.5 hours (most marathons):60–90 g/hr is supported by research, but the practical sweet spot for amateur marathoners is 45–70 g/hr. Above 60 g/hr you need multiple transportable carbohydrates (glucose + fructose, roughly 2:1) because the gut's glucose transporter saturates around 60 g/hr.

Recent work in trained marathoners (Viribay et al. 2020; Urdampilleta et al. 2020) showed that intakes of 90–120 g/hr are well tolerated and reduce muscle damage — but only with deliberate gut training over weeks. This research is the ceiling, not the recommendation for most amateurs. The calculator caps your target at what your experience level supports so you don't end up chasing an elite number on your first marathon.

If this is your first race at this distance

Choose Finish comfortably as your goal and Beginnerfor experience. The calculator will target 30–40 g/hr for a half marathon (about 2 gels for the day) or 40–50 g/hr for a marathon (4–5 gels). That's plenty for a strong first-time effort and is far less likely to cause stomach problems than chasing 80–90 g/hr because someone on YouTube said you should.

Practice your fueling on long runs for at least 4–6 weeks before the race. Take a gel at the same intervals you plan to use on race day, with the same brand. If 30 g/hr doesn't sit well, switch brands or formats (chews and sports drinks tend to be gentler than concentrated gels). The honest answer is that no rate works in a race that didn't work in training first.

Training your gut (for runners chasing more)

The intestinal absorption of carbohydrates is genuinely adaptable. Cox et al. (2010) and Jeukendrup (2017) demonstrated that 4–6 weeks of repeated high-carb intake during long runs increases both transporter expression and tolerance. If you're a marathoner who has run 60 g/hr comfortably in training and want to push to 80–90 g/hr to support a goal time, 6 weeks of progression in long runs is realistic. Don't shortcut it.

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 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

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.
Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(S1), 25–33.
Jeukendrup, A. (2017). Training the gut for athletes. Sports Medicine, 47(S1), 101–110.
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.
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.
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.
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.
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.
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.
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.
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.

Half Marathon & Marathon Fueling Calculator

Race Fueling Calculator

Three fueling plans for the race

Fluid & sodium

Pre-race fueling

Post-race recovery (first 60 minutes)